I learned the technique from this tutorial (YouTube).
Now I'm making a multi-player version of the game, in a client-server configuration. I've decided to have the server handle all physics, including collision.
However, I'm not sure how to implement that. Since the images are drawn client-side, the server cannot access image's data to do the pixel collision tests. I can make bounding boxes on the server side, but how do I detect pixel collisions?
Tech details: I intend to write the server in node.js, using either CouchDB or MongoDB as my database.