Auto-sizing image fields

Hi
 
I have some image fields that I need to be a fixed width and I need the height to be whatever that needs to be to keep the image in proportion and fit the width of the image.
 
I've tried ticking "expand to fit" for the height, but this makes the image field jump up to ridiculous sizes that the image doesn't begin to fill.
 
I'm happy to do write some code that will adjust the height to keep the image in proportion, but I can't find any way of getting at the dimensions of the original image.
 
Can anyone help me?
 
Thanks
 
Dan
 
(BTW - I'm on Livecycle Designer 9.0, but the forum won't let me choose that)