Set canvas width and height before adding image.

According to the w3c specs for the canvas element, it defaults to a
width of 300px and height of 150 px. This causes any image which excedes
these limits to return black pxiels for those regions outside of this
boundary.

http://www.w3.org/TR/2012/WD-html5-author-20120329/the-canvas-element.html#the-canvas-element