I am looking at the robot

« Force compatibility mode in IE8 mktime »

IE7 doesn't like to repeat transparent backgrounds

It adds a gradient to the transparency, even if it starts out solid (I mean, flat, not-gradient). The easiest way to fix, if this is a transparent bg image, extend it out so it will not need to repeat (eg. a 1-pixel png out to a 1000x1000 png the filesize isn't too crazy if it's just that one transparent colour).