You can use the following method to create the Discovery Image:
1. In the Windows Deployment Services MMC snap-in, expand the Boot images node.
2. Right-click the image that you want to use as a discover image.
3. Click Create Discover Boot Image.
4. Follow the instructions in the wizard, and when it is completed, click Finish.
Now a Discovery Image is created and you can add it to WDS now or you can make a Discovery Image CD.
For more information, please refer to the following article:
Creating Discover Images
http://technet.microsoft.com/en-us/library/dd637996(WS.10).aspx
and
http://technet.microsoft.com/en-us/library/cc730907(WS.10).aspx#DiscoverImage