How to pin a URL to OS X Dock with a custom image?
Move/drag the shortcut to the Desktop first
Find an image (on the web, etc.) Open it in Preview, Select All, and Copy.


Open Get Info on your shortcut on the Desktop. Select the current icon at the top left of this window. It will highlight to be blue.


Command-V to paste the image over top of the icon.
Close the properties window.
Drag the shortcut icon to the Dock. Note that items can only be dropped between the trash can and the black divider line.