I have found an answer to my own question called KTPhotoBrowser which fits the bill exactly -- just a thumbnail gallery and photo browser. Combined with SDWebImage which handles the image cacheing it appears to be a perfect simple solution that doesn't require 2 MB of code!
Update: quick to install, works with tab bar controller (example app provided!) and works like a charm. May not have all the features of Three20 but its a lot easier to use and has a much smaller footprint. TabBarController woes made the final decision, I spent several hours trying to get Three20 photo viewer to play nice with my tab controller app -- and failed! After only a half an hour of futzing I got KTPhotoBrowser working by using their tab sample app as a reference point.