Can one pan across a panorama photo that is longer than the screen in Keynote?
What you're looking for is similar to the "Ken Burns Effect", so named for the documentary director that made the technique famous.

Steps to complete in Keynote:


Import an image larger than your slide, and resize as necessary, but ensuring it remains larger than the slide area.
On the Build Inspector, click on the Action tab and select "Move" from the Effect drop-down.
Increase the duration of the effect as desired.
Modify the movement path (represented by the red line over the slide) so that the end point is to the left of the start point, on a straight line.