some of those sites are now also on https.
go through a few of those linked images to get an idea of which sites are linked to the most.
then check if those sites are using https and test a few of their images with their https.
now you have an idea of a large group of urls that you can pass a sql query to, converting them from:
http://example.com/image.jpgto
https://example.com/image.jpgfor the rest of the sites, and other random ones, you can either keep them as they are or risk doing a batch conversion again. this is one dellema which many long running websites need to deal with.