I'll move this to Announcements, where we field technical questions like this. There is a thread in Announcements of people who are willing to design signatures for our members, if you would like to do that.
You can use Photoshop, which is pretty expensive. I have Photoshop7 myself, and you can use Paint Shop Pro, which I also have. Plus there are several other programs you can use. You could probabaly even use Paint that comes with Windows, but you are kind of limited with that one, but for a simple basic siggy you could use that. You would need a program where you can change a BMP image into a gif or a JPG, though.
There is a program called Micrografx Suite, that though not free, is very reasonable in price. I bought that before I got my first Photoshop. By using it, I learned a lot about layering, and resizing, masking, adding special effects and so many other things. The suite is only about $45. and it has a picture publisher, on the same order as Photoshop and Paint Shop Pro, it has a designer, where you can create vector images, and even an animation section in it as well as a way to create 3D images. It's well worth the money. I still use it for some things, like creating castles and cool stuff like that.