It is often not possible to mirror only images, because HTTrack must follow links on the pages (HTML) to find all the images you want.
The good method is to crawl a site with default, or general, rules (stricter rules may be better, but very dependant on the actual site) and use a Build Structure (see Options > Build
tab) like:
Html in web/, images/other in web/xxx, where xxx is the file extension
This way, when HTTrack saves the files to your hard drive, the images are separated into their own folder.
Many online galleries use some form of javascript when clicking thumbnails to produce pop-up windows with the full-size images. The ability of HTTrack to successfully find the full-size images is dependant on the complexity of the site's javascript. Sometimes it will work, sometimes not.