I certainly don’t know all the ins/outs on skulls, but I believe the human skull (or images of it) is not necessarily bad or evil. Have you noticed that a great many paintings of saints contain images of human skulls? Sometimes the saint holds the skull in his/her hands, sometimes the skull is somewhere in the background. The skull, if I understand correctly, is a symbol of human mortality and the fact that we will face death someday and will suffer the consequences of our actions. Some saints (I think but do not know) yearned for what the former owner of the skull he/she is holding presently has: physical death and eternity with a God they had sacrificed everything to live for. I have a pretty good, though inexpensive, replica of a human skull in my home sitting near my books on theology and Catholic Christianity. It doesn’t frighten me and I don’t see anything inherently bad in it.
Of course, I could be wrong.
Peace of Christ,
Roman63