By default, you own every picture you take on Instagram, but once photos are on the service, they're kind of stuck there. If you want to licence your photos so anyone on the web can use them, I Am CC is a webapp that lets you give your photos a range of different Creative Commons licenses.
Photo sharing sites like Flickr have been doing this for years, but since Instagram is primarily smartphone based, it doesn't have a system for directly licensing photos. The benefit of a Creative Commons licence is that you can give any web site access to your photos and they'll attribute it to you.
To release your Instagram photos into the wild, you need to give I Am CC access to your Instagram account. Here's the catch, unlike a service like Flickr, you can't pick and choose which photos get Creative Commons licenses. Instead, it's every picture you take. Thankfully, I Am CC operates in three month cycles, so when you agree to release you photos, only the next three months will be licensed, after that, you have to sign up again.
You can stop the service at any time, but you can't revoke access for photos you've already licensed. If you need a primer on Creative Commons, check out their Licensing options, and if you're willing to release your Instagram photos to the web, head over to I Am CC.