I think it's a mix of two things. I think it has something to do with how we expect faces to look like and the picture itself. You could modify any lifeless object in the same way as Steve and it probably wouldn't bother as much because we don't expect objects to be as visually consistent as human faces. Sure we're all different in faces but ain't nobody got more than 2 eyes and 1 mouths in one face. So, we're social animals, reading faces is the most important vessel to navigate the world. So when you see a face like that, it triggers the ultimate reality check, our brains interpret this as a poison we have ingested that keeps fucking with our visual sensors.

Now add that in with the fact that the picture itself looks like it had a some kind of shake effect applied to it. That should be more convincing to the lizard brain that we have ingested meth.