'Click to Reveal' pictures on Twitter

I'm not quite sure how this is done, but click on this tweet from Sky Atlantic - then click on the picture, and you'll see it change.

Original

After clicking:

Great effect!  See it for yourself here

Another example here

Update - The guys at Tommy know how it's done:  "If you saw this clever tweet from @skyatlantic and wondered how it was made, it was done with a transparent PNG @twitter has a white background and the picture viewer has a black background"