Flickr has taken the relationship they have with Getty Images a step further, making it incredibly simple for any Flickr user to make their photography available for inclusion in the Getty catalog by turning on a simple flag.
Last year Flickr entered a partnership with Getty Images to allow Getty Images to contact Flickr users and ask them if they would like to include their photos in Getty's massive stock photography database. Now they've introduced a feature where you can proactively tag your photos to alert the world and Getty Images you're interested in selling them. When you look at individual photos in your collection you should see "Want to license your photos through Getty Images?"—as seen in the screenshot here. Clicking on the link gives you the options seen below (click for a closer look).
You can enable the "Request to License" feature which will bring your photo to the attention of Getty's editors, it's up to you to decide if you want a public badge on the photo announcing your request. From there out your photo will be available for licensing requests. When someone requests to license your photo a Getty editor will contact you and help you arrange the terms of the licence and the fee. Check out the official announcement from Flickr at the link below for additional information.