Course Thumbnail not displaying

Hello,

When a image is uploaded in the Course Settings for the course thumbnail, the thumbnail on the settings page briefly shows and then displays a image broken link icon. When viewing the course on the All Courses page, the thumbnail area is blank. It is possible to use a colour instead but not the image.

There doesn’t seem to the anything in the Learning docs about the required thumbnail image format or dimensions.

I have tried reducing the size of the image but it doesn’t help.

How can thumbnails be used to show on the Course Settings page and the All Courses page?