A person, usually female, that is only attractive in a thumbnail image. This is similar to HFFA.
Josh is browsing on the Internet when he sees a thumbnail of a girl.
Josh: Oh, she's hot!
Josh clicks on the image
Josh: Shit, she's a thumbnail beauty!
Josh then proceeds to vomit all over the screen, probably in an attempt to shield his eyes from the girl's ugliness.
a girl who is only good looking in the thumbnails of her Facebook profile or even twitter. After enlarging the photo, you find out that the girl is actually not good looking at all.
guy 1: holy shit dude, who's that girl????
guy 2: dude i don't know lets go check her out. she looks hot in the newsfeed
*enlarges the photo
guy1: holy fuck she's ugly
guy2: well shit, that's thumbnail beauty right there