Nah, it's easy.
You've just gotta use bmp or xbm(?) files, 32x32 rgb colour.
I put all mine into the MyDocuments\maya\7.0\prefs\icons directory, since it helps make all your personal setting nice and portable - easy to take elsewhere to someone else's pc.
Then it's just a matter of going into Windows->Settings/Preferences->Shelves. Select the script, hit the change icon button and find where you put em all.
Here's the only 3 I bother with. The Xray two are just resized screen shots of a panel window. The third one's used to make sure the seam of a model sits on the Xaxis - great for mirroring... Dead basic one liner, but so much quicker than fiddling about.