Icons for blocks?

We are planning to roll out a GNU radio software.

To cut the story short, I need to add icons to blocks instead of the
normal rectangle or write the code to draw fancy shapes for blocks.

2 questions:

  1. is an xml tag for image possible? (for ex in cheetah template)
  2. where is the code located for drawing the rectangles? (which I could
    possible manipulate)

Any help would be greatly appreciated.

PS- Great work guys! keep it up!