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1.
term used to describe a person that appears to be attractive in a thumbnail picture on a social networking website, however, the full size image reveals their ugliness.

sometimes used to describe if you would have sex with the person if they did, in fact, look how they do in the thumbnail image.

goes along with the trend of people always placing the image they look best in as their profile picture.
I was browsing Facebook today and ran across what i thought was a cute girl. She looked good in the small picture, but once I clicked on it I found out she was a thumbnailer.
by Linger May 20, 2009
 
2.
Someone who only looks attractive in a thumbnail picture...until you click to enlarge.
Guy 1: Damn dude this chick on Cindy's page looks nice.
Guy 2: Click her and lets see!!
Guy 1: Okay bro *clicks*
Guy 1 and 2: Oh my damn, its a thumbnailer. Tricked!
by CarlRosta July 07, 2011