That is not pixel size... pixel size is "something like" 720x480
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