The design is an optical illusion that will trick the eye into de-synchronizing the movement outside the picture and the movement inside the picture, giving the illusion that the breasts [shitposts] are actually bouncing up and down.

https://knowyourmeme.com/memes/shake-your-screen-optical-illusion

Sciencey source: https://www.ncbi.nlm.nih.gov/pubmed/12854649