Licensing Photos for/from Social Media

  The copyright for a photograph belongs to the person who took it, and for most photo-sharing sites, you retain the copyright while licensing the site to post and share it.  However, without a definitive indication of downstream licensing, the consequent use of the photo may be problematic.  (And no, the fact that a photo is shared online does not mean its licensed to use by everyone under all conditions).
  If you're a professional, or want to be one, you'll need to explore the various options for licensing.  The American Society of Media Photographers has a good online Licensing Guide that outlines copyrights for photos and the various licensing options.
  On the other hand, if you just want to share your photos with others without having to develop specific licensing language, there's Creative Commons licensing language.  Creative Commons (CC) is a nonprofit organization which has developed a set of standard licensing statements that indicate what uses you want to allow others to make of your content, while confirming your ownership of the copyright..  They basically outline three aspects of downstream use to license - whether you want the work to be attributed to you; whether you want others to be able to manipulate (change) the work; and whether you want to allow your work to be used for commercial purposes.

  The photo-sharing site Flickr allows you to set a basic (default) Creative Commons license for all your photos uploaded to Flickr.  It also allows you to change the license for specific photos through the "Owner Settings" option.  More than 200 million photos on Flickr bear Creative Commons licenses.
  There's also a way to easily attach Creative Commons licenses to photos uploaded to Instagram. I am CC is an add-on service that allows Instagram users to sign up under their Instagram account, select the CC license they prefer, and then every photo uploaded to Instagram for the next three months will carry that license.  In a nice move, I am CC asks you to renew the service every three months as a reminder of the license choice.

  If you're looking for photos to use, there are several sites that aggregate images by Creative Commons license type.  There's also a number of useful online guides for how to properly use and reference those images.
  Regardless of whether or not you're a professional, it's smart to get into the habit of thinking about whether you want others to be able to use your work, and under what conditions.  So why not start with the vacation photos you're sharing with friends and family.
  And if you see a great photo online you want to use, save yourself from the possibility of a copyright violation take-down notice, and pay attention to an image's licensing and follow them.

Sources & Resources
How to License Your Instagram Photos on Creative Commons, Wired
I-Am-CC website
Creative Commons images and you: a quick guide for image users, Ars Technica
Using Creative Commons Images from Flickr,  SquidooHQ
American Society of Media Photographers Licensing Guide
Creative Commons website