node image attribute doesn't seem to work

Hi again,

I'm trying to use the image attribute inside a graphviz node, and it doesn't seem to be working:

fszvfs [label="Mr User", image="/var/www/html/File/imagecache/user_image_default/File/userPhotos/picture-51.gif", URL="/user/56"];

In the resulting chart, I can see the node gets larger to accomodate the size of the image, but the image itself is not displayed.