Currently Being ModeratedDec 29, 2012 7:27 PM (in response to 27993buck)
All objects are contained within a Group, even at the top level, including images.
Animate your image as you want (keyframe, Throw behavior, etc.).
Add an Image Mask to the Group level (select the group and type Command Shift M).
Draw a rectangle that will become a "viewport" for your image to "enter" into (preferably within the group with the image.)
Drag and drop the Rectangle layer (from the layer list) onto the Image Mask added to the group (the rectangle layer should become deactivated [unchecked] automatically; if not, uncheck it in the layers list.) [The Rectangle object must be Filled; whether or not you have it outlined is up to you.]