Scale Image to fit in width or leave unscaled

Hello,

I would like to achieve the following image resizing as a general rule in
my docbook5. I generate the docbook myself using python. And finally I
generate pdf from the docbook.

if image width will not fit the page
   it shall be resized to fit in width
else
   image size shall be left unchanged

I am aware of the scalefit, scale, width and content width. But I am having
difficulty in using a combination of those to achieve the desired logic
above.

Paul