The website's code is the background is given as a URL 
(http://(websiteurl).com/img/(image name)).

The mirrored site has the same structure but lists the 
background image as (.../.../external.gif?..(URL)).

If you could give me the right filter, or suggest a free 
replacer to go through the many pages and do the replacing 
for me, I'd really appreciate it since the image in 
question is not external as defined by HTTrack.