10 years into this and just when I thought I had seen everything! When I publish any of my Studio 13 courses, the result (which I preview or then load to LMS and view) has some slide images from a completely different course. In fact, every time I publish I can get a different image from a different course showing up as the background image on the first slide. Most of the time, it seems to draw the image from the just previously-published course.
I tried reinstalling Studio. This did not fix the problem.