Just to be sure it is a bug, do you get the same result with another skin that supports variants?
I can't see which skins do support variants. The description of the Minimal skin is a year old, no variants are mentioned.
But both FancyBox and Tiger do generated just one file playgroun.jpg, while FancyBox see this srcset:
Minimum version 8 is bundled with the latest jAlbum release and not (yet) available from the skin download section on the website. Just delete it from your local skins directory and use the bundled one.