Reproduce:
1. Open a wave with a sketchpad gadget in it using Google Chrome or Safari
2. Draw something in the sketchpad
3. Close the wave and re-open the wave

You should see the previous drawing, but instead the sketchpad is empty.
Currently the code relies on the onload method of Image DOM objects which is 
only supported in Firefox currently. Need to find an alternative.

https://code.google.com/p/sketchpad-for-
wave/source/browse/trunk/sketchpad.xml#20