You can easily do this in Motion. Select the Object and then go to the Media tab in the Project palette (Hit Command-4 or the "Project" button if not already visible). Select the thumbnail related to your image (again, in the Media tab, not the Layers tab) and then go to the Inspector. It will show the different interpretation and timing behaviors of your source footage. Set the Start and End frames to the frame you want and then set the End Condition to "Hold". You can now alter the duration manually in the timeline for your frozen image.