Thought this might help someone -
Noticed today with 10.7.4 shortcuts to internet sites in the dock were missing their spring icon. The shortcuts were there and working but just a blank space in the dock, no icon.
Finally figured it out, looks like the url.png was missing in Lion. I copied it from Snow Leopard to Lion and all seems well now.
This discusses further - maybe related to 10.7.4 update
https://discussions.apple.com/thread/3942402?start=0&tstart=0
