I believe what is meant to happen is that someone starts a .gif file with an object in it. The next person takes the .gif file, and adds an object. In however many frames they want, they depict the first person's object interacting with their object in some way. This repeats. (Right?)
I think this would count as a forum game if it was just a free-for-all, with everyone adding in an object whenever they want and then posting it. For this to be a collaborative project (and for it to work), you'll have to have a set group of people who want to participate, and a deadline of some sort. If you don't have any sort of restrictions, then this will just spiral off into offtopicland.
That is to say, you could have this as a forum game (with some sort of rule to make it a game- like "each object must destroy the previous object in some way" or something), where it will almost certainly be locked at some point in the future, but which might be more fun in the short term; or, you could organize this into an actual project.