I've gotten a lot of questions on how to create wallpaper. But I never added a How-to page because, well, there is no "way" to make them. It's all creativity, there are a million different techniques.

But for starters, this is a very good guide. You can also see this page for a list of programs (I personally use Jasc Paint Shop Pro, however, which is not on the list).

Keep in mind that a wallpaper is an image that goes on your desktop. Therefore it SHOULD be desktop-sized --- 1024x768 or 800x600 pixels. It should be more than just taking a screencap off Firebender.com and adding one word of text in MS Paint. You should at least remove the Nick logo in the corner (I do this with the Clone Brush in PSP), and do some touch-up to the image by adjusting the colors or softness.

After that, be creative. Maybe you can blend two pictures together, so that it looks like Zuko and Katara are standing together. Or you can erase the background around Aang and put some cool effects behind him. You could make a collage of all the Aang/Katara moments. You could even use a real photo as the background, and make it look like a bunch of pictures of the gang are tacked on a bulletin board. These are just a few ideas. Have fun with it!!