eh? The images on a webpage are referenced by their url - eg.
www.domain.com/images/image1.gif - how else can you get an image by not using this url? There's no backdoor.
Google obeys robots.txt so you're kosher MadHatter.
spidersam - I recommend you start again with your robots.txt file, you could be screwing things up and not knowing. Check out -
http://www.searchengineworld.com/cgi-bin/robotcheck.cgi and
http://www.robotstxt.org/ to learn more.
Where did you copy that bit of code from? Doesn't that page explain what you are actually doing?