Puzzle save error – unsupported image?

I installed this plugin and attempted to create my first puzzle. I set the featured image to an existing 400×300 jpg and when I tried to save as draft or publish I get “Featured image is required. Invalid image format. Upload an image correct format, please Back edit page” The page does save to the list of puzzles, but if I drop the shortcode in a post, nothing appears, so I guess it doesn’t recognize the image.

Other than the video tutorial, which has no audio, there is zero documentation that provides any insight as to what image formats, sizes or anything else that it does or doesn’t support. Is there a minimum? Is there a maximum? I don’t know, there is no documentation at all. Some basic information would be appreciated. Thanks.

Rick