Chrome is stretching the height of my max-width image

I'm having trouble with setting the max-width of my images in a CMS I'm using. In Chrome (untested in other browsers yet), setting a max-width stretches the height of the image I insert - see here: http://www.literarykitchen.co.uk/events/testspace/

This is the code I'm using:

#content .pagebanner img { max-width:490px; }

Is there any reason why it's stretching like that? I thought maxwidth meant images scale in proportion?

Thanks

Osu