Single black frame in enhanced podcast rendered in GarageBand.

When a podcast created in Garageband is exported or shared with itunes, the resultant .mov and .m4a are jpeg w/aac and will have occasional black frame at a chapter marker.

The chapters are created by droping images into the podcast track. These tiff images created from grab captures of screen shots. The audio is an mp3.

Anybody have any suggestions or ideas of cause?

OS and iLife are uptodate.