I haven't had any problems with the drag and drop myself lately but I have (because of past issues) for a long time taken the photos I'm using out of my photo library and put them on my desktop before dragging them over, years back I found less problems doing it that way (though I'm not sure if you are using phone or desktop, I always use my computer).
Another fix I've found when something was weirding out with the photos was to "save for later" the listing, then go back to it from drafts, sometimes it looked like my photos had disappeared but lo and behold they were still there in my drafr. Lastly could try signing out and then back it, that sometimes clears out freaky glitches.
