Do you have a specific example?
What version of HTTrack are you using?
Here's a guess...

You can set the background image for a site using CSS.

Example from <http://www.w3schools.com/cssref/css3_pr_background.asp>

"""
body
{
background: #00ff00 url('smiley.gif') no-repeat fixed center;
} 
"""

Until recently, httrack didn't know how to get image links from CSS.

The bug was fixed in version 3.47-12 (May 2013).

<http://code.google.com/p/httrack/issues/detail?id=2>