Just so no one else has to bother isolating the tattoo again... :)

A modder's resource to make adding Geralt's Blue Stripes tattoo to
your own re-texture projects a little easier.  Drag-and-drop the
.png onto the topmost layer of your 2048x2048 diffuse texture in 
GIMP, then set the layer to "darken only" mode at 50% opacity.