The photo appears to have been removed from her Tumblr account, but because nothing ever disappears from the Internet we can see that the note reads:
This is most likely a play on the insulting phrase, “Take that, bitches,” which is generally used after someone makes a slam dunk during a pickup basketball game or makes a really good point during an argument, according to linguists.
“Mitches” are presumably Mitt Romney supporters, who Beyonce is essentially calling “bitches” because they failed to overthrow Obama.
Beyonce probably stole the bitch/Mitt idea from Jay-Z after he altered the lyrics to his hit “99 Problems” from “I’ve got 99 problems but a bitch ain’t one,” to “I’ve got 99 problems but Mitt ain’t one” at an Obama rally Monday.
Or she could be aiming the insult to a group of men all named Mitch. We may never know.