Image preview in Media Library broken after removing cropped images

Removed cropped images from uploads folder because I knew I wouldn’t use them. I didn’t want them included when ftp-ing my uploads folder to the web. Now the originals don’t show up in the Media Library (actually the preview image shows up as a broken image link).

1. How can I get my preview back? (link info still appears correctly in the library and images still exist at those addresses)
2. What’s the best way to delete unused cropped images – or should I not do that?