You can use maxImage to scale images based on the width of your browser, the height, or both. This example sets the 'maxFollows' option to 'width' so that the margin to the right of the image is always the same.
You can view the source code to figure this out, but the HTML code is:
<img src="width_tester.jpg" class="widthmaximage" />
<img src="bg.jpg" class="bgmaximage" />