You're making a new thread on /tg/ and don't have a related image. No big deal, you simply reach into your vast collection of cool images from the Internet, and select something you think would get the attention of your fellow fa/tg/uys. Of course, you add "pic unrelated" at the end of the post, in case people get the wrong idea. You wait for responses, and they come. And then you realize you have done something very very wrong.
You, my friend, have fallen for /tg/'s Classic Blunder: Never start a thread with an image more interesting than the text.
The phrase originates from the movie The Princess Bride, specifically this clip.
Blunder can also be used to describe a bad move in chess.