There might be some ideas in here:
Don't just copy, though. Use ideas to give you inspiration for something original with you.
The following GOOGLE Search might just give you some ideas:
You've seen the traffic signs in a circle with a red diagonal line through it? You could find an image (even a large dog threatening a tiny one) and use that. If you give me a "theme" I have lots of Images & Photos sites.
Thanks both of you for your help .
P.S. I had time to hunt for some images for you:
1. (from Kidzworld): http://www.kidzworld.com/article/488-bully-or-bully-hater
2. (from Parent Dish): http://development.bloggingbaby.com/2006/01/09/discipline-when-kids-test-parenting-mettle
3. (American School Foundation; here's a nice one): http://portal.asf.edu.mx/8182081012154560/blank/browse.asp?a=383&BMDRN=2000&BCOB=0&c=53379&8182081012154560Nav=|&NodeID=796
Can you draw? On the dog theme, perhaps a snarling bulldog and a tiny chihuahua? OR, a very large thumb pinning a tiny person down? Don't be afraid to let your imagination work on this!