You can't use keywords such as right, top etc in conjunction with px or % when using background-position.
However you can position the background using :
background-position: 50px 100px;
This will define the offset of the top left corner of the origin image from the top left corner of the background. The first value is the horizontal axis and the second value is the vertical axis.
To position an image from the right hand border you would need to use percentages. You may have to use trial and error to get it exactly where you want, but a position of 50% 50% will position the middle of the image with the middle of the background. So as I understand it a position of 95% 0% should position a point at 95% of the image with a point at 95% of the background. (Percent causes the appropriate point within the origin image to be aligned with the appropriate point in the background element).
background-position: 95% 0%;
(Note Navigator 4.x doesn't support the background-position at all)
Hope tha makes sense.