Although there is no catch, it's far from perfect and hardly magical. Its accuracy goes up to ~55% on the VQA (http://visualqa.org/) dataset (which is short of state-of-the-art by ~7%).

Did you look at the examples? Many of them are wrong, and others are guessable just from the question (e.g. "What shape is the plate?" I would say "round" without a picture)