On Firefox it was as easy as right clicking on the image and copying the link.
Chrome refused to show the link though, and I would guess others e.g. Safari would be more similar to Chrome.
Solution: Firefox greatly simplifies such things:-D.
Edit: also, I don’t know about whatever UI method you used - an app or the webpage one or whatever. What I did here was to just type out exclamation markm(this is what caused an image to be auto-loaded in the Fediverse), straight braces some alt text, parentheses with the image link inside, then check the Preview to make sure it would load before posting. Like so: