What does it mean when you see a spirit which is a form of a skull over your face in a picture?

Photoshop

It means that your death is short becoming and your life is dwindling because this spirit is taking your life away

Or that someone is really good with Photoshop.

Sometimes the interpretation might be based on the understanding of the individual. A skull is mostly a hidden part of the body. If the hidden skull is seen in a spirit form, perhaps it could indicate that there is a hidden spiritual part of a person that can be explored that they did not fully comprehend, before exploration of it. Since the physical skull holds and protects the brain, the spiritual skull might hold higher consciousness, that has that barrier to it's access.

Alternate view

Spirits do not exist. Therefore you are simply projecting (as in imagining) an image onto something else, like a vague blurry shape.

If you are seeing this image repeatedly and vividly (externally i.e. not just in your head) it is a sign of mental illness and you should seek professional help immediately.