> Problem is that httrack writes this file as "images/our
> logo.gif" while the html-page refering to it still 
mentions
> "images/our%20logo.gif". The result begin that this images
> is not displayed

Err, the %20 things are normally identical to spaces, and 
should be unescaped by apache AFAIK:

RFC #2616 (http://www.ietf.org/rfc/rfc2616.txt?number=2616), 3.2.3
"Characters other than those in the "reserved" and "unsafe" 
sets (see RFC 2396 [42]) are equivalent to their ""%" HEX 
HEX" encoding."

I would also suggest to check the image access rights/owner 
rights.