You don't have much content on the first page so the background is centered with the content as the body height depends on content.
You could set a min-height for html which will allow the image to sit in the center.
However on very long pages your image might not be visible if there is a lot of content so better to used background-attachment:fixed instead.
background:url(images/leaves.png) no-repeat fixed 50%;
Now the image will be central whatever the content.
Note that at 400k that leaves.png is 10 times bigger than I would like to use for a background like that.