Make Category Thumbnail different than Product Detail Thumbnail

We have a client who want to have the product thumbnail that shows up on the Category pages different than the one that is initially shown on the Product Details pages.

I changed the $icon_image_path to $detailed_image_path in the image source in the image template, but I ended up getting the default “No Image” on the Cat page while the Prod Detail page was fine.

If anyone can point me in the right direction, I’d appreciate it! Thanks!