goodroi - 10:50 am on Oct 12, 2010 (gmt 0)
actually you are asking some really good questions.
i have had best results with image search when i host the images in a subfolder instead of a subdirectory.
you should not need to submit sitemap. why not? if you are getting great traffic from image search, why not use all the tools available to you? i would go ahead and submit a sitemap. i would also make sure to focus on backlinks for the entire website.
i know you say you are getting a good portion of your traffic from image search. do you know if that is good traffic that is converting? most websites i work on, the traffic from image search is very poor in quality. of course that depends on what industry you are targeting. just be careful that you are not simply chasing after traffic but instead chasing after profits.