Thank you for posting here and letting us know!
There are a few limitation on the uploader such as it is not able to support upload using multiple tabs and uploading in parallel at the same time.
The minimum dimension of the product photo should be 800px x 800px and the maximum dimension should not exceeds 2000px x 2000px. The product photo needs to be in .jpg format.
At some rare cases, if the uploader detected that there are corrupted images due to data transfer, it will not allow the upload to complete. If you have experienced such incident, please send an email to firstname.lastname@example.org together with the image file attached and we will perform a check for you.
It's our responsibility to make sure that all the tools provided works well, and if you faced any problem, we will definitely make sure that we fix it for you and all sellers.
If you still face this problem in the future, please let us know.
Wishing you a wonderful day!!