You'll want an Image Manimulation Program for this.
I only know one freeware program: GIMP. You'll find downloads when you go to google.com and search "gimp download".
I don't know what other details you could think of ^^'
Oh, and always keep in mind: Keep your sig under 190pixels height. If you're over 190px, it's against the forum rules, and the GMs eventually will remove it.