This error occurs when your listing photos are in the png format, and has a transparent background.
By default, Etsy gives all photos in the png format that have transparent backgrounds a black background. Check this discussion on Etsy.
At the moment, the only way to resolve the issue is;
Once your new photos are uploaded, SynCommerce will automatically pick it up and sync it to all connected stores.
Contact support at firstname.lastname@example.org for any further help.