The ViewPoint property controls which part of an image is visible on the screen. If the image fully fits into the component frame, ViewPoint will be the same as the upper left corner of ImageRect. When an image exceeds the component boundaries, however, ViewPoint indicates which point in ImageRect that is visible at the upper left corner of the component. An image can be scrolled by changing ViewPoint to the desired location in ImageRect. Changing the ViewPoint is done by built in scrolling functions, so you should seldom need to manipulate this property.
ViewPoint is given in ImageRect coordinates. To get the corresponding base coordinates use the ClientToBase function.