Eyes/brain more sensitive to brightness than colour. Black+white border is bright, is updated in the brain quickly, dim image of Patrick is updated more slowly. Discrepancy causes percieved movement.
I noticed my center screen on my dash does this when driving at night. Very perturbing when i notice it in my peripheral vision moving out of sync with the rest of the dash and moving more than everything else.
For the people that may not know what to look for or can't see the illusion, Slowly tilting your phone (or monitor I guess) makes it look like Patrick is slightly bobbing back and forth in the void. Should be obvious what it is supposed to be I guess.
Actually, shaking fast also works just found out. Somehow the image of Patrick lags behind the movement of the border when the brain processes what it's looking at.
Oh yeah, that's cool, the QR code around him began expanding and taking over my entire field of vision. The only thing I can see now besides this black and white void is Patrick on the mechanical seahorse... stationary... beckoning to me to give him another quarter... I must go now.
After a few seconds it looks like Patrick is moving independently from the background border. Basically he just turns his head towards you and opens his mouth like he's screaming. A jpeg obviously doesn't make any sound, lol, but it's drawn in such a way that we all know what must be done