Find any picture/sig you like, upload it to an image hosting site such as photobucket/image shack, and look for the IMG URL. You paste that where it says ‘signature from URL’ (or something similar) in your profile settings.
Well, there’s a lot of different styles/designs and directions you can go. Usually I start with a pretty big image size, like 1000x750 in Photoshop. You get a background, add any effects to it, crop it, etc. Then you usually add text. Your name, rank, server, clan, group, anything you want identified by. You add effects to those as well, add some finishing touches and you’re almost done. You scale the image down considerably, do a few test runs to make sure the size looks good. And you’re done!
If you need any help getting the image into the sig, just let me know.