Some of this is for Premiere Elements, but the idea is the same for PPro or Encore
Pictures that are "overly" large may cause video editing problems
Photo Scaling for Video http://forums.adobe.com/thread/450798
-HiRes Pictures to DVD http://forums.adobe.com/thread/1187937?tstart=0
-PPro Crash http://forums.adobe.com/thread/879967
Right click on the still and check Scale to Frame Size, but chances are you end up with black bars.
You are better off scaling them down manually in the Effect Controle/Motion/Scale.
Do one, copy and Paste Attributes on the rest of the still.
They are not actually zoomed in. They are just too big for the frame. Most cameras nowadays take photos that are much bigger than the HD frame size that they shoot for video.
As Ann says, use the Scale parameter in the Motion effect. It allows you to do it manually, and then, if you desire, you can adjust it a little larger to get just the best part of the picture in the frame, without worrying about going over the actual size. Going up over 100% causes quality issues.
If you automatically scale it to frame size, it resamples it. You don't generally want that.