Lemmy uses markdown for formatting. If you're unsure how to do something, you can look up "how to x in markdown." For example, a DDG result for "how to embed images in markdown":

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

That site has lots of other markdown tutorials if you're inclined to browse. If you want to experiment with the syntax away from the public eye, there are plenty of editors that will show you live what you've created. Many options for doing this locally on your system exist, but so long as you're not working with sensitive data, I like this online option:

https://markdownlivepreview.com/

Also, some (but certainly not all) Lemmy clients have buttons that will generate the right syntax for you.