Setting "img max-width:100%" is a technique employed in responsive/fluid web site design so that images re-size proportionally when the browser is re-sized. Apparently some css grid systems have started setting this style by default. More info about fluid images here: http://www.alistapart.com/articles/fluid-images/

Now, our img element will render at whatever size it wants, as long as it’s narrower than its containing element. But if it happens to be wider than its container, then the max-width: 100% directive forces the image’s width to match the width of its container.