Validate thumbnail dimensions

Whenever you provide an image for the thumbnail, validate that it's following the correct size and format

From the docs:
> The thumbnail should be a .png or .jpg image with a 16:9 aspect ratio. The recommended size is 1920x1080 pixels. Other sizes are also supported, as long as they keep the same 16:9 proportions, otherwise the image may be stretched.
> Some parts of the platform display a reduced square version of the thumbnail, cropped to the central 1080x1080 pixels. Keep anything essential, like text or logos, inside that safe area so that it's not cut off.

Display a small version of the actual thumbnail image in the scene settings, and add a gray layer over the cropped sides to make that clear
Add a tooltip next to the image explaining the required dimensions and the cropping