We are having the same problem. The message that we receive is:
Unable to Submit for Review
The items below are required to start the review process:
There are still screenshot uploads in progress.
We have converted the images from jpg to png - no difference.
We have deleted the images that that said image load error - no difference
We have tried loading on different computers - no difference
We have never had this problem before
At first the images all show as having been loaded. There are no errors. Then we click submit for review, and all the images are greyed, and say that they are still uploading and are being processed. Some get the image loading error. When delete all the images and reload, it is not the same images that fail the image load test.
This gives the impression that Apple does not value our time. There are 10 pages of people who have had this problem going back for over a year. This is unacceptable.