How image orientation influences perception
No life can emerge in chaos - for life to emerge the environment has to be orderly. Organisms have to adapt to their environment, meaning optimise (or satisfice) output in light of input. In a patternless environment, there is no chance doing so.

Our brain evolved in light of this - we constantly learn and update what the "norm" is - let it be there's light if it's day or the hair colour of your partner. Equally, the brain constantly compares stimuli to the norm - if it's all good, no action needed, but a deviation from the norm shall raise an alarm bell.

Thus, we are 'programmed' to ignore what's normal, and be alert to the abnormal. Perhaps the most famous example of it is the sound of mechanical clock, which is simply not perceived once we spend enough time in the room it is in.

In your specific case, seeing the same image for a prolong time results in it becoming a norm in our brain, resulting in reduced brain activity.