I'll bet the problem with your pics is they need to be resized....too big will get that message. I use 600X 800, works perfectly
Already downsized from 4000 X 3000 to 400 X 300 and 72 Points/inch = 60Kb, anticipating a size problem --- this smaller set is the size that doesn't work.
Maybe try this.... Instead of using "insert image" button, go to the "Go advanced" button at the bottom of the page, then just use the "attachment" button... See if that works out better for you?
Same results, either option. I'm doing all this from my desktop, not a mobile device. I'm guessing the "mobile" section is the only one that's working. I'm not being asked to "search" for the attachment, just enter a URL. So when I "drag & drop", there's not a legitimate URL being moved. And that's the error message --- something about a bad URL. If there's a good URL associated with my desktop images, I'm clueless how to find it.