I was having this same issue, and I found a solution that worked for me! What fixed it for me was after uploading the images, I went back to the Product Page Optimization, then back into my optimization with the issue. Now, when I looked through each of the images I'd uploaded, I found one that now appeared as the gray background with a cloud icon! I deleted and re-uploaded that image, and it finally worked! Definitely feels like a glitch on Apple's part. Anyway, I hope this helps someone else!