Some people, when they decide on a design for a tattoo may choose to have a picture of skull tattoo applied to their body. Many tattoo shop customers choose this sort of design especially if they like designs that are a little macabre or odd. Skulls, crossed bones and other scary tattoos have been popular with tattoo shop customers for as long as tattoos have been around.
Many skull designs have other elements added to them to create new and unique tattoo designs. Many tattoo shop customers want to have a skull tattoo to show that they like to live life on the edge a bit maybe. Skull tattoos aren't just for male customers though. Many women have skull tattoos on their bodies, perhaps with flowers around the design or the skull might be smiling or a little more light-hearted than the male customer's version of the design.
Once the customer finds an image that might look nice as a tattoo design, they take that image to the tattoo artist and have a sketch made of a unique design that has the skull image in it.