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1.
pictures easily found on the internet by googling "demotivational posters". The posters are composed of a picture (often funny looking, frightening, or disturbing) surrounded by a rather thick black border, and underneath the picture, in the border there is a title for the "poster" that has something to do with the picture, and underneath the title is some kind of phrase that goes into more detail than the title and makes sense of the picture in terms of what point the creator was trying to get across.
"Hey what are you doing?"

"I'm making Demotivational Posters! Here check one out"

"Oh dude that is so true! if you don't hit that jailbait, somebody else will!!"
by House J.R. March 16, 2010
 
2.
A take off of the classic 'motivational posters'. De-motivational posters are more often than not sarcastic, insulting, or making fun of something widely known, or comical.

Often a picture bordered in black, with a large single word title written in white, and in some cases a comment below written in smaller font.
Did you see that de-motivational poster on 4-chan?
by Furofushi February 21, 2009