I found a program that optimizes PNG images. When I used it on my sig, it dropped the file size about 3 kb, putting it under the 20 kb limit. This might be useful for anybody who has a PNG image they want to use for a sig but it's slightly too big. Download it from this website:
http://www.cs.toronto.edu/~cosmin/pngtech/optipng/

PS. I posted this in the bashpit because I was afraid doggfather would lock it :'(