PNG images, but I can't open them

Here is an example:
https://www.file-upload.net/download-127...0.png.html
Size: 17,6 kilobyte.
File extension: PNG

This is from Orc Castle, a PC game. Apparently, the game uses a special PNG version (?) to hide its graphics. Manual Pictures, cursors, etc. are displayed, but the rest can't be opened.

Is there any trick to it?