We just had a good discussion over at theNXTclassroom.com mailing list about creating custom graphics for your MyBlocks.
It turns out that the icons for the MyBlocks are in .png format and can be found in:
.../Program Files/LEGO MINDSTORMS Edu NXT/engine/Icons/MyBlocks (or remove the Edu if you have the retail version)
I needed to delete one of the existing icons to keep it to 72 files (6x12 grid of possible icons in the editor) before adding my own.
Now I just need to figure out how to make this block actually work! :)
*clipart from openclipart.org