Just ran into the same issue where the user has multiple images to choose from however regardless of what was being selected the same image was being applied.
Here is the answer - http://deployment.cybertechcomputers.co.nz/MDT_Reference/ConfUDITaskSequToDeplDiffOperSys.htm
Essentially just create a task per every image you plan to deploy. Name the task appropriately, select the image, and set the task sequence variable condition for OSDImageName