system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] What's with all the skull imagery in the Imperium?

Seems a bit grim (yeah yeah grimdark I know). They've got that double headed eagle but then many of their units have skulls as their symbol (Space Marines, Inquisition, Imperial Guard, Adeptus Mechanicus)
gpt
The skull represents the sacrifice that maintains the Imperium of man and it represents humanity itself.

It is important to make the distinction: it is a human skull, not some Xenos they killed and used as a trophy, human.

It is used to show that at it's core, the human being is perfect. Skulls are used to display that even in death humanity is superior to any xenos. It also helps making the people of the Imperium accept death as a natural part of life, which makes it easier to lay down your life for humanity and it's Emperor.