Issue with fixed background images in Safari 5.1

I've created a Tumblr theme with a somewhat complex background. It renders as coded in Chrome and Firefox but Safari is giving me trouble. Basically the two background images assigned to the body element seem to readjust whenever the viewport dimensions change and w开发者_C百科hen scrolling. It looks really bad, and I've attached a perfect example of the problem below.

Does anybody know what might be wrong here and how to fix it? I know the code is correct, so I think this is a Safari specific bug. The theme is located at http://blog.danielimmke.com/

EDIT: Apparently this issue happens only on the OSX version of Safari. The Windows version renders as coded.