Manually check all images that your users upload.
If an image is copyrighted then the owner of the image should have put a copyright mark on their work.
- The word "Copyright"
- The abbreviation "Copr."
However if you have a gallery or something similar or the image does not have a copyright mark on it, but you are sure you have seen it elsewhere on the internet, to that where you have to manually check the pictures before they are submitted what you can do is save the image to your computer and then use googles image search facility at http://images.google.com
You can the click the camera icon and upload the image that you want to check to google and it will check all websites that have been indexed by google for similar images. This is excellent and works brilliantly for checking images and their legal status.