As long as it's GPL, it should be fine, but the Forms API provides a #description field that you can put help text into for a given form element. If I were you, I'd look into that before I start sticking images into modules just for input field help.
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Cameron Eagans
Owner, Black Storms Studios, LLC
http://www.blackstormsstudios.com
Hi,I want to include a small image in my module to provide a small tutorial for the module's input field. Is anyone aware if we are allowed to commit an image into the module repository or not?