Motion 5 with image sequence and replicator issues

I'm wondering if anyone knows happening here. I've just been doing Mark Spencer's tutorials on replicators and I'm up to the session on image sequences. I've made my own project based on what I've learnt in his tutorial but I'm coming across some odd behaviour with either the replicator, sequence replicator or the photo I'm using in my image sequence. I made up 18 jpegs from my own personal photographs, most are 1920x1080 but some are 1080 x1920 to suit the framing of the photo. I imported the image sequence into my project applied a replicater and a sequence replicator to the replicator as is the case in the tutorials. I have the photos coming in from out of frame and stacking themselves into a pile, much like a deck of cards. My problem is that the photos that are 1080x1920 are only rendering in half. Does anyone know whats happening here? My computer is an IMAC 21.5, 2.8ghtz I7 processor, 12gb of ram. TIA