I like the video idea....

Hmm... Maybe a Story contest.
Like a max. number of pictures to tell a story, with your dogs in the pictures?

Or some other ideas:

Another best friends
Kind to kids (Dogs with kids)
Best helper (Dogs helping do things)
Impersonate smilies (lmao.. I dunno)
In action (any picture of dog running, swimming etc..)
Dog with toys (Picture of dog with favorite toy(s)