It's actually not that complicated to set-up MORE warnings for all the uploaders.
1. Make everything HTML upload
2. When images are chosen to upload and user clicks "Upload" a big red window should pop up (or a redirect to a follow page) asking: "Are these your own images, or did you download them somewhere else?"
3. If they press: "Yes, these are my images" another big yellow window will pop up saying: "If you upload images that you did not take or create you are committing copyright infringement. If any of your images infringe somebody's copyright, you will be held liable for all damages and loss of royalties that occurred due to your actions". If you still want to continue uploading these images press: "I am not committing copyright infringement" otherwise click on "I didn't know that but thanks for telling me."
Is that so hard?