PowerPoint 2013 has no limitations to the number of slide objects you can add within the slide area. Be it shapes, multiple pictures, or any other slide objects — you can add them all. When you add multiple pictures on the slide you may end up placing them in a haphazard manner and pictures of different sizes may be scattered all over the slide. It is good idea to arrange them all properly to get a more aesthetic looking layout. This tutorial explores how you can group pictures on your slide.
Learn how to group and ungroup pictures in PowerPoint 2013 for Windows.

