Any photo editing software if you're trying to create. If you have a pic you just wanna load up in your sig, just get a free hosting site like photobucket.com, load your pic, and put it's URL between the . I use Photoshop CS2 and Image Ready CS2. If you PM me with what it is you want (detailed description), I can make it for you or at least try.
I would say just right click and properties on on of the sigs on this forum to see what the size is on it and use that as a guidline. In photoshop you can then just click NEW and set the size there. You could also set the size in the script. [img src=hxxp://blah.com/blah.jpg width=400 height=140] It's still would be easier to set the size in PS since you can see the finished product in realtime instead of not knowing till it's posted.