once an image is saved to a bit format it is pretty much cast in concrete.
If you then attempt to scale the image down, portions of the image are often
discarded and cannot be returned when you attempt to re-enlarge the image.
the quality degrades pretty quickly. with a vector format the drawing can be
scaled usually without loss of quality thus easing the move between
applications that support it. the new svg format from adobe is an excellent
means for allowing cross platform and browser based sharing of data
drawings.
al