When you use the same picture for a contact in Address Book on two different computers, a conflict may occur when the Address Books are synched.
Add an image to the contact on both computers (even if it's the same image). When the sync conflict occurs, pick one of the two images to resolve the conflict. Note: It is possible for this issue to reappear.
When a picture is added to a contact in Address Book, the image is stored as a TIFF file. When the image is saved, information from the computer's Display Profile is used to compress the image. Because each computer's profile can be different, this creates an image file with slightly different information. In this situation, when you perform a sync using iSync, the two images are interpreted as different, and a conflict occurs.