Go to open >File and open the image which you want to apply on a pic.Now also you maybe use magi eraser tool
because if you are using an image which is containing background you remove the background very easily by
magic eraser toll.
Now drag you tattoo file which is obviously open in a new window.Drag your file to the image on which you are
applying a tattoo.Now press ctrl+t this is a short cut key of free transform now adjust your tattoo image to the image where you applying a tattoo.Now goto your layer pallet you also decrease opacity to 80% and then below in the layer pallet you see the option of normal change this to luminosity you all work done and as you see below tatto is apply very easily.
That’s all done and you see how to apply a tattoo in Photoshop very easily and quickly.