Most of the Citrix Provisioning deployments I come across leverage 2 basic mage types. Standard and Private. Very high level, Private images are read write and are typically used in a 1 to 1 model between machine and image. Standard is a read only image that can be used by multiple machines simultaneously. Production deployments leverage standard images for large deployments of users. Since the vDisk is in read only mode, any change you make to the machines using the standard image will be lost upon reboot.
In order to update the Read Only vDisk, you will need to do the following:
On the Provisioning Server, locate the directory that has the standard image you would like to update and make a copy of it (VHD and PVP files). The LOK files are not necessary as they are LOCK files.
After creating the new vDisks, you will need to add them to the Provisioning Server Console Store.
Once added to the store, you can edit the FILE PROPERTIES and change from Standard to Private.
With the new vDisk now in the store, you can assign it to a VM, power it on and begin modifying the image.
After making the proper modifications to the vDisk, you should power down the VM and repeat the process of switching the vDisk back to Standard mode. Next step to to assign the new vDisk to all of your machines. Even if they are currently on, they will pick up the new vDisk image at the next reboot.
For HA environments, be sure to copy the new disks to all appropriate locations as well.