Hi MadridMike and welcome to Crystal XP
I'm sorry but there are two different docks available
but as for your title, I think you're using Object Dock with Vista Inspirat.
So here is what you have to do,
Right click on the dock item you want to change the icon and go to 'Dock Entry Properties'
Then in the dock properties window, you will see a button named 'change image' (Just below the current image preview) click it and a change image window will appear.
There, chosse 'browse' to navigate to your desired icon or png file and clcick OK. then Ok all windows and you'll get a new icon for the item in your dock 
In need of help just upgrade this thread 