Mary Magdalene has been painted dozens of times with a skull.  It symbolizes the contemplation of death, and wisdom.  The head or skull also symbolizes wisdom or Sophia.  The book, something else MM is often painted with,is an even stronger symbol of wisdom-Sophia.

Here are several depictions of her with a skull, print any one of them that suits you and place it on your altar.  This is optional, but drawing the skull symbol (“dead head”) in ashes is required.

Penitent Magdalene by Reni Magdalene by Reni Another one by Reni Another Magdalene with skull by Reni Magdalene by El Greco Magdalene with skull and book (symbol of wisdom, Sophia) By El Greco by Fetti Magdalene with book and skull, by Fetti