You'll need a image editing program. MS Paint works fine for me. For a free image editor you can use gimp, which is compariable to Adobe Photoshop. Another interesting tool is Wally. It's originally for Quake textures, so the graphics use the quake pallette, but I still think it's a really usefull tool.
Here's another one you might want to check out: World Creator. Their old version (1.5) is freeware, and it looks like a pretty cool program. I haven't had a chance to use it yet, but it's on my near term list of programs to mess around with.