If you have a solid color background to your slides this can be done using a shape and the Action "Move". For example, lets say that your slide background is white, you will need to:
Add a square shape and make it big so it covers the whole picture, with extra on every side and set the fill to white with no line
Add a circle and make it the size of your "spotlight" and give it a black fill
Select the white square and the black circle and copy (command c or Edit>Copy)
Open Preview and make a new from clipboard (command n or File>New from clipboard)
Copy this and paste back into Keynote
Now use Instant Alpha to remove the black circle
Select your newly added shape with the circle cut out and "Send to Back" (Arrange Menu or right click)
Then select the picture you want to spotlight and send it to the back +(this will put you picture at the very back with the spotlight over it, and any text or other pictures in front of the spotlight.)+
Then in the Inspector>Build Tab you can add Actions to move the spotlight to show the part of the picture you want to see.
