Ignore if you already know this, but I was having issues when I joined a few weeks ago so passing on what I learned.
Unlike reddit, Lemmy lets you edit your original post so you can still add the image if you wanted. Either the direct image link from wherever you are hosting or you can upload to whatever Lemmy instance you're on. I had trouble getting images to upload until I learned there was a 100kb limit on lemm.ee. It will just fail to load without providing useful error message unless your under your servers file size limit.