Previous drafts said it's possible, by using:
border-top-image
border-right-image
border-bottom-image
border-left-image
border-top-left-image
border-top-right-image
border-bottom-left-image
border-bottom-right-image
However, the current candidate recommendation for CSS Backgrounds and Borders(25 November 2009) these properties disappeared. It looks like you cannot use different images for each border side anymore.
As a personal note, I wouldn't have used different images anyway to avoid flickering for the same reason one would pack button states (unpressed, pressed, hover) in a single image file and then use css to offset in the source image.
EDIT: border-top-image
, border-left-image
, etc were present in WD 2002-11-07 but disappeared in WD 2005-02-16.
EDIT2: You might be interested in the jquery.borderimage.js plugin.