Theme Preview showing preview of CURRENT theme

I’ve set up many wordpress sites and this is the first time I’ve come across this problem. Also couldn’t find the problem in support. Oddly, the Theme Preview shows a picture of the current (active) theme when I click on a new theme. The title in upper right Activate link fits the new theme and it will switch to the new theme if I click activate, but the preview picture is of the currently active theme. Any idea what’s going on with that? I re-uploaded the wordpress files but still getting same problem.

Any help is very appreciated.

Christy