eBay - How to turn off image sync off and manage images directly on eBay listings:

To do this:

1) Click Customize grid:

2) Scroll right and check 'Catalog Image Sync'

3) Return to the XpressGrid and scroll right on the grid to find the image sync column:

4) Toggle image image sync as required either on individual products or in bulk

OFF means eBay listing will NOT be updated from your ecommerce platform images i.e. you can change on eBay directly and it won't be overwritten. 

ON means images are being synced from your ecommerce platform to your eBay listings (any changes directly on eBay will be overwritten).

 

Amazon

For Amazon you have to turn off syncing "Listing details".  This stops image sync which allows you to manual adjust on Amazon directly in Seller Central.  Note, turning off syncing listing details means you also stop syncing title, description etc.  (For a full list of fields click 'show' and scroll right).  There's currently no ability to only turn off image sync.