I think most useful would be a nice looking, yet simple "badge". Don't start with massive animations or whatever. Keep in mind SFML is a basic framework, it's far from being a complete engine (think Unity or Unreal Engine).
In the end, ideally it should be good enough to say "peeps, use this, don't come up with your own, so we're recognized", but for something like this it has to fit into other environments, no matter the art style, and it should be simple enough so people can use it easily (would you want to bundle a 5 MiB AVI file with your 500 KiB?).
It should be a "lean and mean" asset people can add to say "thank you". It wouldn't be forced/mandatory, so it has to be attractive.