For reading/modifying/creating TSK themes, try the free
ThemeGenCE program. I believe one of the members here might have written it.
The Amit over at SBSH suggested
thislink as an example for creating icons using either Adobe Photoshop, Paint Shop Pro, GIF Movie Gear or IconBuilder XP 1.0.
Myself, I often like to play around with
The GIMP as it's free and does most of what I as a novice need it to do in the manner of Photoshop.