The Image Indicator, per se, does not have a Scroll property (as far as I am aware). However, as with any other LabVIEW Indicator, it will respond to Mouse Events when the mouse if over the Indicator, including "Delta" (which is -1 each time you scroll down one, and +1 for scrolling up). You can then (programmatically) turn this into a movement of the scroll bar(s), getting as "fancy" as you wish (too bad there isn't a "mouse orientation" property to allow you to turn the mouse 90° and have the wheel mean "Horizontal" scroll.
Note that you can directly query the Mouse, but it doesn't work well. My current Mouse says it has 3 axes and 8 buttons, but when you look at Axis information, they are Horizontal, Vertical, and Scrolling, the first two doing what you expect and Scrolling doing nothing at all. Similarly, only 4 buttons show up on the "button info" display, left-click, right-click, and wheel press (tilting does nothing, and I didn't get the fourth indicator to light).