Final answer:
Swivel is the most useful tool for viewing 3-D images as it allows rotation of the model, providing a comprehensive perspective of the object.
Step-by-step explanation:
When viewing 3-D images, the most useful image manipulation tool would be Swivel. This tool allows the user to rotate the 3-D model, which can provide a better perspective of its structure and details. On the other hand, tools such as Pan allow you to move the image position without changing the viewing angle; Zoom lets you move closer or further away from the object for a more detailed or wider view; and Scroll moves the viewport up, down, or side to side within the image plane. However, none of these tools gives the interactive three-dimensional control over the view of the model that Swivel does.