It doesn't change. The image that you are clicking is a transparant png with white stars and the black lines makes up the room that can be seen in the preview version. The element background on the preview site is white, whilst the element background on the clicked image is black. Thus creating the illusion of a changing image.

Everyone else did a good job of explaining it so here is a gif

Yeah like others have said, it's the same image - just using transparency to take advantage of how Twitter display the same image in two different contexts.

An easy way to see what's going on is to change the background color of the div holding clicked image to white.

It's a transparent image.

That's an insightful technique! Pretty cool.