> I have a pretty dynamic website.  It accesses all the 
> images via a /images.aspx page and all the HTML pages via 
> a /index.aspx.  HTTRack can't detect the MIME types 
> returned by the /images.aspx pages -- these MIME types 
can 
> be image/png, image/gif, or image/jpg.  It seems that 
> HTTrack can only deal with hardcoded MIME mappings and 
does 
> not properly read the HTTP header that is returned in 
> response to a request for the /image.aspx content.

Set options / MIME types : erase all default MIME types.
This should do the trick..