1) "Look at the pretty pictures I took during my last vacation!" (Really, they're pretty craptastic, but I'm glad you got to see a beach with a sunset, just like everyone else who does a presentation.)
2) "Look at my child doing something funny!" (You know, this is the picture that your child will hate you showing all of his friends when he grows up.)
I really have to wonder, why do they feel the need to do this? Seriously, it's distracting from the topic. Stop it. Put in pictures that actually matter to the topic, or nothing at all. I care not about your personal life while you talk about the latest and greatest software.
Of course, while I typed this, the current presenter put up a picture of the head boss in a pimped out outfit, so that was actually funny.