Details in this area have changed over releases,
and it depends on exactly what you mean by
relative (e.g., relative but with leading path
segments or pathless?), but try setting your
graphicspath (see help topics 495, 425, 171 and
others) to point to the directory holding your
graphics. If you do that, I believe your graphic
filerefs should be pathless which might be what
you want.
There is also a graphic_relative_path ACL function
(help topic 1571) if you find you need to do some
ACL programming to get what you want.
paul