Screen with the youtube feed

Some following up comment

I didn't want to put "play" triangles on the thumbnails, because I rarely see people doing this last time (and because it usually looks ugly).

But it'd still be nice to additionally designate a pic as a video with something.
Sadly, the solution has come to me later. And it's pretty simple one actually. You just put a timecode in the right bottom angle of a thumbnail (like YT does). Say, a rectangle dark-dark-grey with a white text on it.