auto image resize on hotspots interactivity page

Hi
I need guidance on how to change the (presumably) css parameters that control the size of the image panel on a hotspots interactivity page;
I want the image to auto resize when I maximize the xerte page to full screen
At the moment it doesn't so I'm assuming the panel, or placeholder or whatever is fixed pixel size;
So can I re-write the css to a % size maybe?
I have experience of writing css but can't find the relevant css file in the xerte folders that would control this that I can tinker about with
Any help gratefully received
Andrew
Southampton UK