solid_color_thumbnail

An image whose thumbnail is simply a solid color rectangle.

This can happen in a few situations:

An animated image or video whose first frame is solid black or white, which causes the thumbnail to be solid black or white too.
An image with a transparent background that was intended to be viewed over a black background, but the transparency was converted to white for the thumbnail.
A thumbnail surprise (a two-frame non-looping animation where the first frame is intentionally blank).