?3d64b1 is added to each URL

Something is causing my URL’s to change and I have ?3d64b1 at the end of nearly every URL on my site. This is the cached version.

In the cached version the images with ?3d64b1 at the end of the URL are not appearing and are causing a 403 Forbidden access error. Images that do not have ?3d64b1 at the end are showing correctly.

Can you please tell me what to do to stop this.

Thanks!

http://wordpress.org/extend/plugins/w3-total-cache/