I encountered this while running an ASP.NET WebForms app using the ASP.NET Development Server.
I suspect something similar will happen if you use IIS Express as your server as well (VS 2010 SP1).
I ‘resolved’ my problem locally by editing the project settings (under Web) and changed from the ASP.NET Development Server to IIS on my local machine. I can see that PNG was already defined correctly as an image MIME type and indeed when I hit my local IIS server it’s serving up the file with the correct type.