Hmmm. Ok I
think I see your problem. If you originally drug the site
directly to the dock, you cannot modify the icon from the dock location.
To change the icon you need to do that before adding it to the dock-- in other words drag it to the desktop or some other location first, then select it and use the command+I (or select it and use "File-->Get Info". Change the icon, close the Get Info window and then drag the modified item to the dock.
One downside to that is that you cannot delete the original after it is dragged to the dock- as now the dock icon is really only an alias to the original item's location, so that is why I said "...drag it to the desktop or some other location..." above. If it is on the desktop when you drag it to the dock, it will need to stay on the desktop. It would be neater to put a "Dock Icons" folder or something on your hard drive and place it there--- then change the icon and drag it to the dock.
Oh yeah, that is
real clear.... sorry- it makes sense but is harder to describe than do!

The only alternative I see is to stick with the original icon, but what I have tried to describe is not difficult really.
EDIT: OK just to show you it
does work, I just made one and randomly stuck it in my dock!
