Process.Start cannot open .PNG file

You need to associate an application which will, by default, open .png files.

This can be done from Control Panel.

For example, in Windows 7 go to:
   Programs -> Default Programs -> Make a file type always open in a specific program

then scroll down to and select the .png extension, click on Change and select a default application such as Windows Photo Viewer. 

Alternatively, if you don't want to make a  permanent setting, then use the Process class to run the .exe you want to use and pass it the path to the .png file as an argument.