A lot of clipart sites I've seen have strict rules for using their graphics.  Give credit, never change them, etc...  I say take the graphics and run!  Enjoy them as you like.   All that I ask is that you use graphics from my clipart site, and not from my other web pages.  I'll be tinkled pink if you email me or write something in the guestbook to say thanks, but you don't have to. 

(But if you do let me know that you're using my clipart, I'll link to your site.)

And remember - Have fun!