Using a graphviz snapshot from May 2011 (2.29), the following syntax
digraph g{
imgnode[image="apple-touch-icon.png", label=""];
}
results in
You’ll need to set an empty label to prevent the node name from being displayed.
If this is not working for you, you may check the output of dot (something like dot -Tpng -o graph.png graph.gv) – if dot can’t find the image, you should be able to see an error message.
I did not have to specify any size (I think this is for svg).