custom thumbnails not working with WP Gallery

I am using the built-in WordPress Gallery. It’s making thumbnails from my images. I want to override them with custom thumbnails. I thought the way to do that was to upload the custom thumbnail to the Media Library then paste the URL where it asks for it, where you edit the page. But the custom thumbnails are not showing up. I just want to know how to get the custom thumbs to work. Should I change the cache folder permissions back to where WP cannot make thumbnails? I’d rather not do that as the site will have a bunch of members, some of whom might want that feature enabled. But if that’s the only way, I will do it. Thanks for any help.