A head must have at least one colored pixel, so it can't be completely invisible. But to make part of a head (or any texture) transparent, follow these steps-
If you are using GIMP scroll over the Colors tab on the top bar (where File, Edit, etc. are) and go down to Color to Alpha. Choose whatever color you wish to be transparent from the menu and click OK/Finish when you have made your selection. Hope this helped.