I've personally done this for years for both friends and myself as a means for "previewing" tattoo ideas. You'd be surprised at the number of people who had never even considered it! It can also be done as a prank: I am currently awaiting my mother's response to my emailed pictures of the "new tattoo" that I supposedly got this week.
It only takes a few minutes and is easily done by even the Photoshop novice!
*Image editing software such as Photoshop or Gimp (I used Photoshop 7.0)
*Have a digital camera or a photo of the subject you want to "tattoo"
*Image to be superimposed as a tattoo
Step 1: Gather the images
I chose a piece of artwork gleaned from Mark Haddon's art blog of a toilet monster, something I deemed suitably shocking- yet not entirely unbelievable- to my mother and father.
Next, take a picture of the body part you wish to "modify" in your image editor.
Step 2: Begin the process
Pasting the image you want to become the tattoo will create its own layer- make sure that it's above and not below your background photo.
For the "blending option" select "Multiply" from the drop-down menu (Directly below the "Layers tab")
Starting to look pretty good already, eh?