Background image given width of parent element

I use some transparent PNGs on my site and have noticed that one of the background images is 100% wide (resizing the browser window resizes the image as well). I checked the code and found that the image is a background of the #header element, which is inside the #page element. The #page element has a width of 100% which leads me to believe this is what's causing the problem...

Another of my images is the background of the body element, so in the PNGFix options, I've just set 'html' as the class which seems to work for all images in the site (just in case this has anything to do with it...)