I found it totally infuriating trying to link photos to this forum I had at Dropbox .

Here's the solution:

After you copy the public Dropbox link to your clipboard, just change ?dl=0 to ?raw=1 at the end of the URL.
If you leave the link as ?dl=0, although it will look like a link to an image, it will really link to the interstitial non-embeddable Dropbox page that frames the image or invites people to signup for Dropbox.