The CgBI file format, thus named for its extra header, is Apple's extension to the PNG image format. More details about the format can be found here:
http://iphonedevwiki.net/index.php/CgBI_file_format
http://iphonedevelopment.blogspot.com/2008/10/iphone-optimized-pngs.html
I would really appreciate it if Paint.NET can open such "optimized" PNGs, either by itself or with the help of a plugin, so that I can save them as standard PNGs instead. If it helps, here's some C code I found for a utility called pngdefry that does the job, but having the ability to open such images in Paint.NET directly would be simply great. (Technically speaking the images do open, but the colors are all messed up.) Thanks!