Lemmy uses Markdown, so for almost any formatting you want to do, just look up "how to x in markdown" on your preferred search engine. For example:

https://www.markdownlang.com/basic/images.html

(This should work for both GIFs and static images)

If you're worried you messed up formatting, the Connect client offers a preview for your image (and actually a button that will generate the syntax for you), but there are other anonymous preview engines as well. I like Markdown Live Preview.