Thank you both! I did read the manual, but there's a difference between reading and understanding...
I have now reviewed it and actually what I was tryng to achieve is get the images to fit on the screen, so I adjusted my CSS to add those max-height and max-width values for <img> instead. This works! I can see the code inserted in the appropriate class which now happens to be .calibre4, and I now understand what is happening!
Apologies for a noob question...