Early pictures of saints.....Gerard, Rita, Gabriel, Alphonsus etc often had a human skull on a table beside them.  These same pictures printed since Vat II have the skull removed and replaced with a lily or flowers.

These skulls were not meant to be ghoulish and priests and nuns in the 1800's (and earlier) and prior to Vat II often had little skulls carved from ivory, bone or wood attached to their rosaries to remind them that life is short and then comes the 4 Last things....Death, Judgement, Heaven or Hell.

From the Merriam Webster Dictionary
Memento mori literally means "Remember you must die". The early Puritan settlers were particularly aware of death and fearful of what it might mean, so a Puritan tombstone will often display a memento mori intended for the living. These death's-heads or skulls may strike us as ghoulish, but they helped keep the living on the straight and narrow for fear of eternal punishment. In earlier centuries, an educated European might place an actual skull on his desk to keep the idea of death always present in his mind.

It apppears to have been a very Christian practice.

We are now fortunate to have these little reminders made of stone available at our chapel to attach to our rosaries.

Skull and bones used to be attached on the crucifix under Jesus' feet.  It was to also show that Christ had his crucifixion where Adam was buried.

Skull beads can be purchased on Ebay .  Anyone who can make a rosary can put these beads on a head pin to attach to your rosary.  Most shipping is free.

Just search for skull bead white