You didn't press both at the same time, did you? =P
Z goes into "Zoom mode" or something, which basically gives you the image on a black screen. If you never entered that mode before, it should show the image unresized starting at the top left, and then M (maximize) should give it an imitation full screen mode. The size of the image in "Zoom mode" is retained.
If you have a newer version than 6, it might work differently. Well, whatever.