scary teeth
Personally, I realize all dogs show their teeth from time to time. Even when their just playing. A friend of mine had a dog that would show it's teeth every time you would pet him even when he wanted to get affection from people. The dog just did it...
Most dogs I've seen playing however, hardly ever show their teeth. So most people will automatically think that the two dogs are being aggressive. Doesn't really bother me, especially when you show them without showing their teeth. If you had a banner with just the scene of them playing and nothing else, I would get the impression that you want people to know how vicious they can get...but that's not the case here.
Maybe just one showing it's teeth instead of both. Then it wouldn't imply that the two are fighting.