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