There are some quality pictures there already, that would be lovely by themselves.
For some months, we could have more than one dog, but with a forum with 13k+ members and a couple hundred ACTIVE members, there are going to be dogs that are going to be left out.
What everyone should do is pick their favorite photo of their dog(s) and submit that photo. Don't just pick any photos or a multitude of photos, just pick ONE and submit that one.
You need to pick your photo carefully and be sure that it is available in high resolution.
There can be a fold out, or the cover (?) can be a collage of all photos.
I do like the idea of a fundraiser, however, Jess, I think it was, brought up a good point in that we all live in separate areas.
Do some searching in your local area and see how much it is to print out a calendar, this may be the cheaper option compared to a printing company.
I agree with Saje, quality should be over quantity.