You can place the folder of images here: "Documents\Stardock\ObjectDock Library\Images"
You do have to change them individually. Follow these steps:
1. Right-click on the icon, and select "Dock Entry Properties"
2. Select "Change Image"
3. Either browse in the folder tree on the left of the "Choose a new image" dialog box, or use the search box in the upper right. If all of your images are in the same folder, then using the folder tree is easy enough. The search box, however, can be exceedingly useful if you know the file name or part of the name.
Hope this helps. BTW- once you get into OD+ and make it fully what you want it, you won't be sorry! And you'll NEVER go back!