When a foreground image is selected, the "Center Horizontally" and "Center Vertically" buttons are greyed out. They shouldn't be, IMO.
Looks like those buttons are attached specifically to a larger group of text field options and not intended for images at this time. You're right, though, that it would be nice to have a way to center an image on a slide.
In lieu of that button, the arrow keys (if you're using Windows) are a pretty precise way to line up boxes pixel-by-pixel. Centering a text box the same size as your foreground image should give you a frame of reference as well.
