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As the onload is called asynchronously, you need to use a callback function:

1. preload: function(source, callback) {
2.       var proto = new Image();
3.       proto.onload = function() {
4.             var w = proto.width;
5.             var h = proto.height;
6.             delete proto;
7.             callback({width: w, height: h});
8.       };
9.       proto.src = source;
10. }