percentage-based vertical positioning dependent on block width?

I seem to have found an issue when firefox is positioning blocks based on percentage values and margin offsets. in short, look at this page: http://akos.maroy.hu/~akos/frontend/experiment/awake.html the intended size of the image section is 75% of the available area, both vertically and horizontally. the intended location of the image section is so that the it's in the middle both vertically and horizontally. while this works horizontally, it seems that the vertical position of the image depends on the horizontal width of the window. if one changed the width of the window, the image will shift vertically.