That's fine! It's meant to be a .png file! I know, that's an image file but all of the information for the game is actually stored in the .png file.
You will need a copy of Pico8 to play the game and download the .png file into your carts folder. You can then use the 'load' command followed by the filename of the game file to load the game and then use 'run' to run the game.
Hope this helps somewhat but feel free to contact me if you're still unsure about something!