Add ability to edit main product photo after it is added

Countless times, I've clipped a product to the library with all the details, and the main image field defaults to the placeholder image rather than the actual product image. When I go back to edit the product, there is no way to edit the main image, so I have to re-clip the whole thing again. I need the ability to replace the main product image (not just add additional images). Or at the very least, I need the ability to make one of the additional images the main image.