Include, e.g., mydog.png in a project and make the album. The slides directory shows mydog.png. Now go to the advanced image settings and force the core to produce a JPEG, instead. You end up with both mydog.png and mydog.jpg in the output directory. The only way to get rid of the now-unneeded image is to delete it manually.

Then, if you delete mydog.png from the project entirely, one of the slide versions is still sitting in the output.

A fringe case, but just for the record....