YouTube videos have different resolution default thumbnails to use. We use the "default" version (ie: http://img.youtube.com/vi/UNIQUECODE/default.jpg) to preview youtube videos before showing them.
The problem is that Google PageSpeed then dings the page as having a non-compressed image. (ie: Compressing http://img.youtube.com/vi/UNIQUECODE/default.jpg could save 1.1KiB (23% reduction). ).
Is there anyway to call their API and get a compressed image to show?