Silly query I know but I wondered if anyone could help me with a powerpoint problem?

I'm making a random name generator for one of my year 8 classes and they've all made me a colourful/crazy/wacky slide with their name on it, plus various other images... the idea is that I can hit 'pause' and whoever's slide comes up is the student who has to answer the question (it's an assessment for learning thing!).

The problem I'm having is that rather than cycle through each slide smoothly, the slides are jumping at different times, evening though I've set each slide to transition automatically at '0' seconds...

Can anyone help? Does it have anything to do with the fact that some of them may have used bigger images than others?

?????