Scroll in Keynote prestentation/prototype

I use Keynote to put together an interactive web prototype based on images and clickable boxes which link between my different slides, in other words basic Keynote prototyping.

A problem I'm experiencing though is that the height of the images differs between different slides. This causes the zoom level of the images to differ in order to keep the image within the border of the presentation. Ie. it looks like the "website" is changing width as you click along, not very elegant.

Is there any way to get around this? A way which doesn't force me to use the same height on all images. I believe what I need is the ability to scroll in a presentation and also force the presentation to have the same zoom level on the images no matter their height.