To change the file extension in Windows you will need to:
- Go to Folder and Search Options item in the folder menu (the location of this will change depending on what version of Windows you are using, it should also be accessable through the Control Panel).
- Click on the View Tab.
- In the Advanced Settings Tab, untick 'Hide extensions for known file types'
- Click OK.
- Now with the extensions visible change any extension that appears as .PNG to .png
- Load the game and your images should now display.