why won't Ipod touch 3rd generation sync and display some JPG photos?

I exported a group of JPG's from Lightroom into my ipod photos folder. When I sync, some of the photos go on the ipod, but others do not, and there is a message while syncing that some of the photos were not imported becasuse they cannot be displayed. 14 out of 22 had this problem. What is going on? Anybody know how to fix this?