How to customize standard icons in Media Gallery app

Deleting my original solution as it is an unsupported fix. In order to do this properly, you would need to create a custom file store to keep your new images in, and create a new default file viewer to handle rendering of your custom images.

Right, that sounds like a major exercise probably not worth the trouble. I solved this for the time being by switching the group's view type from thumbnail to list in the group's management options. Thanks for the input!