You're going to laugh at how easy it is. (in a web browser anyway)
Copy an image to your clipboard (Ctrl-C or Command-C)
Go to your browser with Lemmy and click in the comment window.
Paste (Ctrl-V or Command-V)
The image will be updated and a link will be pasted in the comment window with the correct markup. You can see the image before you post it by hitting "Preview".