This entry, I use photoshop to stick the tattoo to the back of the girl:D.
These are two original images:
http://www.worldofstock.com/slides/PMO3806.jpghttp://images.elfwood.com/art/z/i/zimmi/tatoo_warg.jpgAt girl image, I use clone stamp tool to delete the characters on her image.
At tattoo image:
Firstly, I click Ctrl+A to select all image, and use Magic Wand Tool to selecte the white areas outside and inside the tattoo. After that, I have selected area of tattoo. Then, I use Move tool to move the tattoo to the girl image. Now, I close the tattoo image.
Secondly, I choose the tattoo ( now the tatoo is at layer 1 of girl image), adjust the its size as fixing the back area of her. When feeling it is fixed, I click OK.
Next, reduce the opacity of layer to 25%, choose the tattoo and open free transform pop-up, choose Warp. Now, I adjust the tattoo naturally as much as possible until I feel that the tattoo merges with the shape of her back.
After that, I click on the indicates layer visibility icon to hide the layer 1, and use Polygonal Lasso Tool to selecte the area around the tattoo.
Then, I click on the indicates layer visibility icon again to make it appear, and open feather selection pop-up to reduce the feather radius to 2 pixels.
Now, increase the opacity to 100%, open Hue/Saturation pop-up, click colorise to set the single color for the tattoo. Here, I choose blue color, so I set the Hue=223, Saturation=27, Lightness=0. When finishing, click OK.
After that, I open Brightness/Contrast pop-up to adjust to make the tattoo to fix the skin of her. Lastly, I set the blending mode for the layer is multiply. Ofcouse, you can choose any mode you want, it depends on you. Just try to play around with them.

PS: Hope that it is useful for you.
Cheer!