Trying to cut out the poor quality pages like image pages from being indexed and therefore harming the rest of the site. I could put those sections in robots.txt to prevent them from being indexed completely, but couldn't I also put the images as a sub domain? Therefore I would still get traffic from Google images etc but the rest of the site wouldn't be lumped together with the poor quality pages. Does this make sense or am I way off?
If i understand this correctly; you want to stop bots from indexing poor quality image pages? Yes, you can prevent that through a robots.txt file. If you want images to be indexed then make sure that folder is not included in your robots.txt file also.