fluid images distorting in responsive theme

Hi,

I am setting up a responsive theme called ‘portfolio-press’ and the image in the posts are not scaling proportionately on a phone or device.
css markup:

img.size-full,
img.size-large {
	max-width: 97.5%;
	width: auto; /* Prevent stretching of full-size and large-size images with height and width attributes in IE8 */
	height: auto; /* Make sure images with WordPress-added height and width attributes are scaled correctly */
}

Thanks in advance,
Rob