They wouldn't be useable as full image backups, but if the data had not been encrypted yet, then that data is stored somewhere in the image and can be recovered

They wouldn't be useable as full image backups, but if the data had not been encrypted yet, then that data is stored somewhere in the image and can be recovered, albeit perhaps with a lot of work. If you use thin clients or at least require all files to be stored on a centralized server, then no matter how many hundreds of PCs were affected, you'd only have to go through on set of images.