[Plugin: NextGEN Gallery] Slide-shows jumping

I have two NextGEN slide-shows in the site I am setting up and they both jump as I click for the next image. The photo jumps and the “next” link also moves. Has anyone any ideas?

It has just occurred to me that maybe the photos need to be the same size? As they are old archive photos this would be difficult.

Please excuse me, I’m new to WordPress and not code savvy or anything very savvy really. So you’ll need to be explicit! 🙂

Thank you.