Welcome to Sitepoint, Scolpy.
I was having a similar issue years ago. I am afraid, if I am understanding your issue correctly that in CSS2.1 you just can't to that. CSS3 promises to address this, but right now support is so limited it's not worth mentioning it as a solution.
If you are positioning the bg image to the right and need space, in PX from the right edge, you can use a GIF/PNG add that # of transparent px to the right of the image, if your BG is a solid color, you can always match the BG color as well, in which case you can also use JPGs)
Of course, if you know the size of your container, you can just do math; position= container_size- (size_of_image+right_offset).
For now this is the the tried and true way of getting around this issue.
Hope that helped a little, at least