CodeProject has published an article I wrote describing the architecture of Gallery Server Pro. I first wrote it for the 2.0 release several months ago, but just updated it for 2.1. It has been very popular, with over 90,000 views and an average rating of 4.6 out of 5!
If you are interested in a behind-the-scenes look, check it out. I describe several of the software patterns, including the composite and strategy pattern. In addition, I describe the HTML rendering model and the data provider infrastructure. There is also a section that describes the image metadata extraction technique.