This is usually done when the person tagged in the photo is drunk, partially naked, making a horrible face, or all of the above.
Verb (it's what you do)
1) The act of removing one's self, usually represented as a label or a tag, from a facebook picture that either depicts the individual under incriminating circumstances or being inebriated while doing something humiliating.
2) The act of coining a weird word like "detag" just so you can sound even remotely cool. Well, this person in particular is cool, regardless.
3) This might actually be a weird Facebook jargon I'm completely unaware of. So...my bad!