Facebook now allows to add short videos instead of a static image in the banner.
Example: https://www.facebook.com/exorstudios/
From what I have read (I am not 100% sure of these details), it can be 20-90 seconds long and should be at least 820x312 pixels (but maybe Facebook resizes the video to fit the banner dimensions).
I just think that you could also add a nice battle video on your https://www.facebook.com/play0ad/ page.
PS: avatars can also be (7 seconds) animated GIFs.