Unfortunately, .Net's image processor for png won't do any optimization heuristics on the output. Your best bet is to have an optipng/pngcrush binary available on the server, render the resized output to a temporary file, then use pngcrush on it via a System.Diagnostics.Process.
For the most part, if the uploads are photographs and the original format is JPEG, then you will be better served by using JPEG output.
