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?
Step 3: Tweaking the effect
I then used the eraser tool to remove some parts of the tattoo, around my real tattoos, the nipples, and parts that were going off the edge of my body.
Step 4: Final touches
I also chose to soften the edges of the tattoo by setting the eraser tools opacity to around 50% and gradually erasing away some of the outmost edges of the tattoo layer in several strokes.
Finally, I blurred the layer slightly by applying the "blur" filter in Filter > Blur > Blur
For this particular tattoo emulation, I stopped at this point. However, more curvy parts (such as arms, butts, or whatever) you could go even further by applying a distortion filter to selected parts of the tattoo layer in order to make it appear as though it curves over the bodies surface.
Save the file as a .jpg
Step 5: Utilization
If you are doing this as a prank (as I was in this particular instance) I recommend a smaller image size as the final product, as its less easily detected as a fraud. Email to prankee in the most nonchalant way you can manage.
Hope folks find this Instructable fun and useful! I recognize that this isn't the only way of doing this- so please feel free to post any suggestions in the comments!