When I went surfing for a clip art image I kinda need, I entered "subject clip art" in Google. I got over 131,000 sites (only a few with the subject clip art, the rest with those three words on the page) I came across a site discussing clip art copyrights:
http://www.infotoday.com/cilmag/jun01/kennedy.htm
with some good information for your review.
The claim that no one in their right mind could find any resemblance between your crappily hand-carved stamp and the high quality original clip art may not be too valid :-(
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