If you don't want to shell out for Photoshop or Illstrator you could go
Gimp or
Inkscape. :)
Also if you like 3D as I do,
Daz|Studio is a great free alternative to Poser, lots of freebies abound for both programs as well.
Flash has a nice alternative in
LiveSwif.
I've used all but Gimp and found them easy to adapt to.