Pixastic doesn't work properly in Safari

Question

I'm using the Pixastic javascript library to make several images on a webpage grayscale. It works in Chrome and Firefox, but not in Safari. Sometimes it makes a few random images grayscale, and sometimes it doesn't do anything at all.

Here's a test page to demonstrate the problem: http://www.edwards.nl/new/test.html

I'm using Safari 5.1.1 on Mac OSX Lion.

Does anyone know what the problem is? I couldn't find a solution anywhere yet.

Edit: It appears that the canvas elements are created, but the grayscaling part doesn't work.