[pedalpete]

You may be able to spread more virally if people don't enter the recipients details into your app, and it may make it easier on everybody.

This is definitely a "maybe", you'll have to do the research.

If you just have the recipients phone number and the photo, you can send a photo MMS to the recipient saying "x bought a printed version of this photo with PigeonPic. We need your address to send this photo".

Just an idea, you'll need to work on it, but I know, I don't have people's mailing addresses.