I may be too late to this party, but I have faced the same issues for the past several hours and here's what worked for me.
NOTE: use images without transparency (JPG) and keep the names simple like 1.jpg 2.jpg. Make sure that all the file names are unique.
1) log out, clear cookies, log in, and remove all images from all devices. Make sure you only have one tab open.
2) upload the images one by one per device. No DnD and no multiple uploads.
3) once the images are uploaded per device, right click on the image and view its url. Copy the url and open the url in another tab. You should see the image.
4) refresh the page after 2 minutes and make sure that you can click on the image thumbnails and see all the images you just uploaded per device.
5) repeat steps 1 to 3 for all other devices.
Don't know why, but this somehow this worked for me. Just submitted my app for review after 5 hours of redoing the same thing over and over again until I tried the steps above.
Thanks!
