Since I can't find a thread on this with the search function (If there is one, do what you want with this one), I figured I'd try my teaching skills out.

Please note that I'm using The GIMP 2.4.0-rc3 for Linux, but these should easily translate to other versions and OSes, just keep in mind your menus and tools may not be named the same or be in the same order as what I say. It'd also be great if someone can port this for photoshop. I think GIMP and PS come with most of the same filters.

First off, open the GIMP. After the splash screen goes awayyou should see at least one window; a tall skinny one. If you have other windows open, you can close them, since we won't be using them. We'll call the tall skinny window the "Main window". Click File in the main window, then click new. You should get a new window titled "Create a New Image". Ignore where it says "Template" for now, and skip to where it says "Image size". The current standard Resolution for a monitor is 1024x768, but I always make my wallpapers 1280x1024, so that the wallpapers look relatively good on most monitors. For this image size, enter 1280 in the text box labled "Width" and 1024 where it says "Height". Then click OK.

Now you should have a larger window with a large white space in the middle of it. The white area is the drawing area (most people should be able to figure that). We'll call the window with the drawing area the Drawing Window.

(I'm sorry if at this point it feels as if I'm testing your intellegence, but please keep in mind that I'm not that great a teacher)

Now we need a background image. We'll go ahead and make one for this.

The first thing we need to do for our background is cover the entire drawing area in black. Make sure you have Black and white selected for your foreground color and background color respectively. Then select the bucket fill tool (It looks like a paint bucket) and click anywhere in the drawing area. The drawing area should turn black. Now we're ready to make the background more exciting.

In the Drawing window, click Filters, highlight over the "Light and Shadow" submenu, then click "Supernova" You should get a new window tiled "Supernova", which should have a small preview of our image and some sliders, in addition to other things. Leave the Supernova center where it is (128x128), as well as the color, but change the sliders to: 100 for "Radius", 239 for "Spokes", and 7 for "Random Hue". Then click OK.

We now have a drawing area that has a bright light comming from the top left corner. We're going to make this look more realistic by adding a lens flare. Click Filters, Light and Shadow, then Lens Flare to open a window titled "Lens Flare" Make sure the center of the flare is at 128x128, then click OK.

Now, our bright star thingy has a nice flare on it. Since the highlight of the picture should NOT[ be the background, this is all we're going to do on it. It would be wise to save the image at this point.

Next, we're going to add an external image to the wallpaper. I think Dark from DN Angel would fit well with the color scheme of the star-thingy, so we'll find an image of him to use. For the sake of keeping the tutorial consistent, I'll put the image in this post:
http://www.lauraharms.ca/images/Dark Profile.jpg
Just right-click it and save it to your computer.

Next, open the image with GIMP. We now have a second Drawing window open with our picture in it. First thing we need to do is scale the image so that it fits nicely into our pic. Click Image, then Scale Image to open a new window with a couple of text boxes and some drop-down menus. You should note that if you change one of the top two text boxes, the other one changes as well. Just change the "height" box to 1024. next, we want to flip it horizontally so that it goes with our background. Click Image, Transform, and Flip Horizontally.

We're almost ready to add the Dark image to the background, but we still have a couple of things left to do. First, click view, zoom, then zoom out until you can see the entire Dark image. Now, click Layer, Trasparency, then "Add Alpha Channel". Now, and I'm not sure if 2.2 has this feature, but my GIMP has a tool called "Fuzzy Select Tool". It may also be called "Magic Wand Tool". Select it, then click in one of the White areas in the Dark image. It selects the white area so that we can delete it easily. Now hit Ctrl+X to delete the white section. Do this for any white in the picture except for the bit on Dark's clothes, eyes, and teeth.

You should now have Dark on the right a checkerboard-looking pattern where the white was. Go ahead and save this image, just in case, then press Ctrl+A to select the entire image, and Ctrl+C to copy it to the clipboard. Then close the Dark window.

Now, click in the Background window and click Layer, then New Layer. When the new window opens, just click OK, then press Ctrl+V to paste the Dark image into the new one.

Now the tricky part. Use the move tool (which should automatically select itself when you Paste the image) to align the right, top and bottom sides of the Dark image with the corresponding side in the background. Once that's perfect (which isn't hard if you're careful) click the Eraser Tool. Below the area that lists all of the tools is an area where we can set options for using that tool. Set them all to the following; Opacity: Leave the same; Brush: Circle Fuzzy; Scale: 10; and leave everything else below those the same. Next, carefully go over the left side of the dark image with the right edge of the eraser tool until the left side of the Dark image fades out into the background (If you don't get it on the first pass, start at the top and go again, but this time, move the brush a little to the right).

Again, now is a great time to save, especially since we're almost done.
After it's saved, select the text tool (for me, it looks like an "A"). Before we add the text, we're going to make it readable. First, select a decent text size, like 60 pixels (px). Also, if you want, change the font. Then, where the color selection boxes are, There is a small bent arrow that points to both boxes. Click it to switch the Foreground and Background colors. Now white should be our forground color. Next, click somewhere beneath the bright-star thingy, and type some text you want to appear on your image. It can be anything, just make sure it's well below the bright part so that it's readable. I'm putting "Who says Dark is Bad?" in mine, but I'm terrible with comming up with creative text that goes well in this type of wallpaper, so you can put what you want.

Here's my finished version:
http://fc01.deviantart.com/fs20/f/2007/310/4/1/Dark_Wallpaper_by_Mysterian077.jpg

This would be better with more pictures, and stronger descriptive writitng.

Pics are comming, but I suck at expository writing like this.

This could be useful to some.
Personally I believe that with so many people making desktop backgrounds for fun i'll never need to make my own ones as it will always be easy to find ones that i love. Right now i'm using one from the wallpaper source thread and before that I got one from a Final fantasy website. But most often I'll find a picture I like and save it to my computer, then just right click and set as background.

i think i might try this...when i get some free tiem.

What's up with all the artifacts around his hair? = \
Either GIMPs magic wand tool sucks, or you had it set wrong. Or you cut the guy out of the picture first, and then blew him up. In which case, the white frizzies are explained. I'd suggest blowing up the picture first, and then cutting the character out of the background. Then you won't have that white mess all around him. ;)

And my Magic Wand tool's tolerance is set at 32, just for reference. (It lifted the character cleanly out of the picture.)
And here's a helpful link on perfectly lifting images from their backgrounds, in cases where the Magic Wand Tool just isn't getting the job done for you :3
http://www.pegaweb.com/tutorials/separation/separate-image-from-background.htm

Cool tutorial besides the artifact problem though. :)

Magic Wand tool for extraction!? Use quickmask, you fools!

Quick mask wut!?
Teach me masks, quick!

Edit:
Just looked it up. I think the paths way would probably be easier for me though, as I have no coordination. X3
And the masking tutorial mentioned feathering the mask; the paths tutorial mentioned splitting pixels instead, so I don't know which method is more precise. (Sounds like paths are, but the path tutorial never said any pixels were split, just that they could be. So Whatever!)

Magic Wand tool for extraction!? Use quickmask, you fools!

Why? It's a pain to use. I used to use it then I discovered vector selection tool.

I find it funny how M always considers people who do things differently from how he does it fools or idiots... :mellow:

I find it funny how M always considers people who do things differently from how he does it fools or idiots...
I think the "fools" part was just for fun. ;)
The masking way seems good actually, for when the MW isn't doing the job.
And what's the vector selection tool? That sounds promising too!

Can I remind you all that this is a tutorial for n00bs. Not your typical computing student. Yippy.

Can I remind you all that this is a tutorial for n00bs. Not your typical computing student. Yippy.
What's your point? Are you saying that noobs are too retarded to read half-page-long tutorials? = \

Vector, MW selection, mask, gimp, OSes, PS px

Maybe if I opened the file thing and tried it myself but for now it's many shortenings and not quite on topic words.

lol, if you read the whole topic, most of the abbreviations would have apparent meanings.

The ones that don't have apparent meanings can usually be googled. The one's that can't be easily googled have been linked to.

We're just supplying extra knowledge for people who want to learn. People who can't spend twenty minutes reading to learn something new really have no use for photoshop or art in general anyway, as they'll never have the patience to make anything decent. > <