How do I control picture size in a slide show?

[PowerPointPeter]

The slides come out ok in the 'viewing mode' but when starting
the slide show only part of the picture is visible.
Zoom control seem to affect the size of the thumb nail pictures only.

 

[John Wilson]

Most likely your picture is larger than the slide and hangs over the edge. In
edit mode powerpoint changes the view to compensate. Try selecting the
picture and shift dragging a corner to make it smaller until you can see the
slide edges.

If this is the problem you should try to insert smaller pics as dragging
them smaller does not decrease file size