what is the best photo editing program to make a custom head texture with?

Well, it depends of your skills. You can try out Gimp, which is free, or Photoshop, which isn't free but really "professionnal".

Good luck

GIMP if you're poor and wish to stay "legal" www.gimp.org ;o

PS for most flexibility.
Adobe illustrator is great for anime style though
It makes wonderfully clean lines and some designs that i wouldn't have thought of with just GIMP.

Too bad my AI trial is over though...

You can get 30 day trials with Adobe CS4 so i'd try that first to see how you like each one
Most of the successful texturers use PS