The only rules I'd like to see adhered to are as follows:
(1) Please name the game. In the title, in the post, whichever. Yes, it may be obvious to a lot of us, but it just keeps things simpler for everyone.
(2) Specify if it's from a photo mode or real-time.
(3) Please be careful with quoting long/large image-posts. It's gonna be choppy enough in here for mobile users as it is.
The format's down to whatever the person contributing them wants to use. That being said, things like the Lynda.com tutorial for how to easily produce WebM content might be worth a look for those of you thinking about contributing media.
Imgur has a very good WebM converter, though in my experience it can have extended down-time. Whatever your url result is, you can add ".gifv" or just ".gif" to get the webm or gif version, respectively. Saving those files to your own hosting is recommended, because eventually they recycle the links if they don't get traffic.
Edit: FasTRapid gave the following format structure:
Switch out the parentheses for brackets, and the code will work. You halve the load times, since WebM users only see WebMs and GIF users only see GIFs.