For this Error:
Buyers love large photos that clearly show the item, so please upload high-resolution photos that are at least 500 pixels on the longest side.
The Solution is:
1. Go to Settings > Selling > Image Options
2. Change Minimum Size Large AND Minimum Size Medium to 500 and Save Settings
3. Bulk upload action=edit with the affected guids and their media1-10 make sure to check Import Media URLs
4. Retry Channel Relist