Lemmy works on markdown, so if you want to do anything, just look up “how to do x in markdown.” For example:
https://itsfoss.com/markdown-images/

If you want to learn more generally, there are many guides, e.g.:
https://www.markdownguide.org/getting-started/

And if you want to play around without other people seeing any mistakes you make, there are multiple web interfaces as well as local editors that will display your works in real time, such as:
https://markdownlivepreview.com/

Hope this helps!

edit: Also, markdown is fantastic, but its syntax can be confusing or hard to remember. I don’t know about all clients, but mine - Connect - has buttons at the bottom of the text entry area to generate the syntax for images (and links). Yours might have something similar.