Have you heard about "Google Image Labeler" yet?

Google Image Labeler is like a game to make Google’s image search better. It’s a fun game, where you work with an online partner, assigned by Google, to look at the same image and decide on some labels together. The Process works like this:

• Sign in to Google Image Labeler if you like to keep a record of your contributions and tell about your identity. If you like to remain anonymous, you don't need to log in to your Google account to label images.
• Google will assign you a random online partner.
• You and your partner (someone in world) will be shown/given same images.
• Both of you have to label/tag them as many as you want.
• When both of you come up with the same label, you both get the points (depending on the labels) and you both move on to the next image. You may also both pass.
• Continue labeling images for 2 minutes, at the end, you’ll get a score based on how many labels you made. However, you can make a track of points only if you were signed in.

With the tags assigned by you and others, Google will come to know which tags according to you should be associated with an image and Google will consider that tags/labels for that image. You can also click on the image (after the game is over) to know from which website that image was?