I've looked at about half of my contacts photostreams to see how much use the CC license (at least in their newest 3 images) and I came to the conclusion that it's less than 20%.
So, I'd like to introduce to the rest of you: The CC (Creative Commons) license!
Using this license for your pictures, it grants everyone some rights, that's why it's also called "some rights reserved". You can pick from 2 simple choices what you want to allow and it will create a license that you can attach to any of your work and allow some usage of it. The basic and most open license is called "Attribution" and allows commercial use as well as creating derivate works that can be relicensed by any license. Only your name must be supplied when your photo is being distributed. The most restrictive CC license is called "Attribution-NonCommercial-NoDerivatives" where one is allowed to distribute and share it as long as the name of the author is given alongside the photo. However it doesn't allow commercial usage and it isn't allowed to modify or build upon this work.
I release most of my pictures under Attribution-NonCommercial-Sharealike which means people can use and distribute my pictures for non-commercial purposes as long as my name is mentioned. They're allowed to modify or build upon the pictures, but they must use the same license for any derivative work created. This is what the license will look for Germany: http://creativecommons.org/licenses/by-nc-sa/2.0/de/deed.en
I also don't upload full sized images to ipernity, or would prevent download of those. I feel that I want more control over what happens with the original sized photo, but want to give something to the community and CC is a good way of doing that!
Give CC a try too!