Select a number of images.  Click on the Slide Show button.  Move the mouse. 
Now, a small number of slideshow controls will appear:  "Restore Normal View",
"Image Info", "Back" and "Forward".

Right now, it requires mouse movement to use these Back and Forward controls. 
Both for reasons of accessability and efficiency, it should be possible to use
the Left (Back) and Right (Forward) arrow keys to activate these two controls.

When the slide show is paused (see Bug #142933), the arrow keys should _not_
unpause the slide show.  With the show paused, and using the right and left
arrow keys, it would be much easier to compare two sequential photos.  My
camera, for example, makes it possible to "bracket" a shot.  This means that it
records three images of the same subject.  The first is underexposed by one
stop, the second is "automatically" exposed and the third is overexposed by one
stop.  The pause function in combination with these arrow keys and the delete
function (see Bug #142934) would make it very easy to quickly keep the best
exposed of these three images and to delete the other two.