I would recommend a Terms of Service in keeping with the service acceptable by your hosting site. Some images may be against THEIR TOS so you both have to be in agreement as what can be posted on your site.
Copyrighted images can get you in lots of trouble without a disclaimer of some kind. Better to set up a way to immediately contact you to pull any copyrighted material if found.
Maybe something similar to what I use:
Disclaimer: By downloading this file, you are stating that you will not use the image linked above for commercial use or for monetary gain of any kind. Removing the trademark of the author is not recommended, and I am not responsible for any legal issues that arise from doing so. While the image may or may not be registered with the copyright office, this does not have any bearing on the status of copyright. While the image is on public display (i.e. on a website, myspace, etc.) please follow the artists requests for giving credit to that artist.
Not being a lawyer, I think you might want to call your hosting service and ask them.
I agree with Ron that a Terms of Service is a good idea. If you are providing a website-as-a-service, a TOS is very necessary for you protection. There are boilerplate terms of service out there. Look at other image hosting services. Use them as your model.
Also, review the DMCA takedown pages offered by video and hosting sites. If someone sends you a takedown notice, comply immediately.
Without a TOS prohibiting users from illegal activity and without a DMCA or similar removal option, you will expose yourself to legal liability.