I've read .img can be compressed or not, but they're generally interchangeable with .iso.

Why would an archive site contain both versions of the same release?

I do not run Tails but it looks like from the website that an ISO file is provided for installing to a virtual machine as well as booting from an optical disk. The img file is strictly a disk image.

By convention (from my perspective) an ISO file is an image of an ISO9660 filesystem that is burned to an optical disk. Modern ISO files like those provided to install a distribution are hybrid ISOs which have additional "stuff" so they can also be written and booted from a USB drive. Technically an ISO is an image file but not all image files can be written to optical disk. Many may use ISO and img interchangeably but IMHO they are not the same.