Because that's what the images really are; here's a direct link for reference.
What you have is a transparent background with some black and white spots on it. The body is given a background-color of plain black for starters, so normally it'd just be boring black. The images have transparency, which means that all of the transparent pixels change to the color of the element they're in. This means that they would normally load as transparent, and then change to black. However, the default background-color for a page is white, so when the background-color:black style doesn't load properly, the transparent pixels change to white, which is also what happens when you click the direct link; the page is white background by default, so the image looks white. If you copy that image and open up Paint, which is unable to process transparency (except emulating it), it changes the transparent pixels to black, which is what would happen if the image were placed on a black background. Paste the image into Paint, and it looks like the regular background because the transparent pixels are black. If the background doesn't load black, it loads white, so the background image appears white.
TL;DR: It's a body style error.
[Edited on 05.08.2012 11:02 AM PDT]