WooCommerce replace product edit screen thumbnail

I am currently using external image sources for my products on WooCommerce.

So i would like to replace the default thumbnail / image across the site.

I have managed to do this everywhere with the corresponding hooks and filters, apart from on the the main products admin screen.

I have the below function:

This to a degree works, it adds the new external image as a thumb, but it also leaves the default placeholder in (to the right of the newly inserted image).

Is there something i am missing? I thought adding a filter replaces the original code of that function / hook?

Any help would be greatly appreciated, Thanks!