An image will fail to upload if the file name cannot be associated with an ASIN or variant.
Some of the reasons due to which we may reject an image are:
The system did not recognise the ASIN (10-digit Amazon product identifier) or variant.
The EAN/UPC/10 or 13-digit ISBN/JAN/GTIN does not belong to an ASIN.
Your ASIN was not yet set up in our system for these external IDs to map.
For more information, please see the Name your image files help topic.