I was able to post photos to the Photo Gallery Testing, start to finish, with an iPad mini using safari. I just tried to replicate the problem mentioned above. The only time they wouldn’t upload after seeing them “preloaded” in the attachment manager window was when no text was entered into the “Title” and “Description” fields.

The pictures would go into the Photo Gallery Testing area no matter the pixel dimensions. However, I had to resize them before they could be inserted into a regular forum reply. I reduced the size with an app before uploading to the Photo Gallery. I used 1200 pixels for the height if the picture was in portrait and 1200 for width if the photo was in landscape mode.

I hope this helps.