> I need to know how to set HTTRACK to download only image 
> files?
Images files + html files, I presume? HTTrack won't be able 
to detect all images if you don't download html files 
(where all links are written inside)

You may then consider using filters (options/scan rules) 
such as:
-* +*.gif +*.jpg +*.png
or
-* +www.yoursite.com/*.html +www.yoursite.com/*.htm +*.gif 
+*.jpg +*.png

depending on your needs.