After you have found the image you would like to tag your friends on you need to right click it and click “Save image as…”. You have now saved the image to your computer. Now go to your Facebook, click “Photos” link on the left sidebar and then click “+ Upload Photos” button in the top right area. Upload the image from your computer and after it has been uploaded open the image and find “Tag This Photo” link below the image. Now you need to click the part of the image you wish to tag your friend on and start typing your friends name. After you have tagged all your friends don’t forget to click the “Done Tagging” link to save your tags.
How To Tag Friends