I can give examples, but it's likely something simple like that. Cheers.
EDIT: Not so simple. Take this page as an example:
https://i.imgflip.com/1ya5du.jpg
- Using the whole-page url works, which was a surprise to me as there's a lot of other information on that page, but the image tags seem to grab the right bit.
- Right-clicking on the image itself and copying the image address (Chrome) gives the same url, which we know works:
https://i.imgflip.com/1ya5du.jpg - Clicking on Show embed codes gives you another https link, that's almost the same, but that doesn't work in Public Press (just a blank post). There's no file type in this, which is presumably relevant.
https://imgflip.com/i/1ya5du
So, current rule of thumb is it has to be https and have a file type specified. Sound right?