Over the years the Punisher has worn a variety of different skulls.  Each artist puts his own spin on the "death head" worn by Frank Castle.  Thanks to Mike, below we have a great collection of the different skulls you can download and print out in almost any size you want.

About this collection:    These are all in high-res PDF files.  You will need Adobe Acrobat to open and print these images.  They come in two formats.  The tops ones have a white skull on a black background.  The bottom set is just the black line artwork on a white background.  You can download a key plan here.  Below the skulls is a PDF file of some of the various logos Marvel has used over the years in PDF format also.