I got a message from Dropbox that my Public folder is no longer public but a private folder like any other. This affected all the image and download links from Dropbox I had in my many posts.
I've now edited nearly all the links I could think of, but there may be some others left. If you're missing anything, please let me know.
If any of you guys need to convert your own Dropbox links, note that the normal links now end with '?dl=0' which opens up a new webpage on the Dropbox site and doesn't work with direct download nor with image embedding. The solution to that was to change the end of each link to '?dl=1' (or '?raw=1'). This isn't obvious to most, and not well documented I believe.