Question: I have some images in png format, I am wondering whether there are .net library to process images?

Answer: 
You may take a look at GDI+ ([System.Drawing](http://msdn.microsoft.com/en-us/library/system.drawing.aspx) namespace) or [WIC](http://en.wikipedia.org/wiki/Windows_Imaging_Component), both have managed wrappers built into the .NET framework. For example here's an [interesting article](http://weblogs.asp.net/bleroy/archive/2009/12/10/resizing-images-from-the-server-using-wpf-wic-instead-of-gdi.aspx) comparing both methods for resizing an image. Of course depending on your specific requirements and what exactly do you mean by *processing images* there might be also third party libraries.
The [System.Drawing.Bitmap](http://msdn.microsoft.com/en-us/library/system.drawing.bitmap.aspx) class supports BMP, GIF, EXIF, JPG, PNG, and TIFF. You can at least display your PNGs using that.

Also I found a library [AForge](http://code.google.com/p/aforge/) which is used in an [image processing](http://www.codeproject.com/Articles/9727/Image-Processing-Lab-in-C) on codeplex.