These are all at least a couple of years old according to captions/descriptions:
http://www.flickr.com/photos/kchrist/1343900919/
http://www.flickr.com/photos/geekmom/522815191/in/set-72157601621988718/
http://www.flickr.com/photos/geekmom/1024308679/
http://www.flickr.com/photos/audsmo/3515149787/in/pool-whiteinktattoos

I have a tattoo idea and I’ve been tossing up whether to use white ink or scarring so I’ve been checking out a lot of pics. The main downside to white tats I’ve seen is that a lot seem to either turn yellowish, blotchy or fade so much you can’t really see them at all. Especially if, like me, you want the design in a place that will be exposed to the sun a lot.
Scarring downsides seem to be that it’s harder to get a clean, smooth line. And because it depends on how your body heals you might end up with scars that look completely different to how you wanted them to. And some parts might scar more deeply than other parts, making the design uneven.
I think your Harry Potter design would look good as a scar, and it’s a simple design so it would probably be pretty easy.
Good luck with whatever you choose, and post pics when you’re done! 🙂