It’s strange, I downloaded a gif to my photo and selected it as an image for a Memmy post. The post worked and was upvoted so others must see it. But the actual gif is not visible on Memmy. The gif does play on a mobile browser though.
I want to try posting a direct link to a gif and see what it looks like. Can you see the gif below?
It looks like you need to use the format:

such as: 
Such a valid point; success should not be measured only in dollars. I began explaining the basics of the Fediverse with some of my friends and the first question they asked was, “How do people even make money on there then?” It’s a bit disheartening to see how money-driven things are for some people.