iTunes does not copy some photos iTunes refuses to copy some photos to my iPod with the following message:


  Some of your photos, including the photo “somefile.jpg”, were not copied to the iPod “iPod” because they cannot be displayed on your iPod


Note that loads of photos are not copied. How can I know which photos were not copied and why? They can be displayed perfectly on my mac.
Apple Knowledgebase article Issues when syncing photos to iPhone or iPod touch recommends deleting the iPod Photo Cache if syncing photos of a known compatible file format results in this message.