You can link them in the body (or even in comments)
You use this format:
For example
And it auto detects and displays the image? Eternity makes me do this to display the image 
yes. I don’t generally add the alt-text option (that’s more for hyperlinks. it doesn’t show on image links). !.
Most websites will let you right click and image and copy the image’s URL, alternatively, you can open that image in a new tab (just the image,) and then copy that URL, and it’ll display the picture. Lemmy will host an image if you’re adding it from your computer. (for that use the ‘upload image’ tool in the editor buttons.
edit, Relevant XKCD:
Use this power responsibly… or not. this should fun.
