1. The link must point to an actual file, not a temporary location as you find when you do an image search. The file extension must be jpg, gif, png, and so on.
2. The link must be https in order for it to work with ProBoards. Http will work if the site with the image file accepts https requests.
3. The shorter the link text, the better.
4. Note that some sites will add a bunch of crap after the image id which controls size and whatnot. In most cases, you can just delete all that so that the file type in the last thing in the image address.
5. And just in case you're quick on pasting in your link info when using the ProBoards image tool, note that you may have have a spurious http:// at the end of the link.