[Help] 2D Shadows/Image Modes

I was watching https://www.youtube.com/watch?v=4BtjcH-iLR0 and he was explaining how a 2d shadow system would work.
I made a little image using minecraft textures in gimp and used the image mode ‘Multiply’ on it.
-=Without Light=-
[spoiler]
http://www.infinitecoder.net/games/without.png
[/spoiler]

-=With Light=-
[spoiler]
http://www.infinitecoder.net/games/with.png
[/spoiler]

-=Remember=-
Remember, these are just images, not the real game.

-=Need Help With=-

  • Texturing a light
  • Using the image mode ‘multiply’ to draw it

If you can help, that’d be great!