Creating pngs bounded to example size

[Mike Solomon]

Hey all,
    I'm trying to create pngs of a small example (say \relative c' { a b c
d} ).  Is there a way for lilypond to make a png that is cropped to a
bounding box so that there's no extra space around the typeset area?  I've
been playing with using different page sizes to accomplish this & with using
lilypond-book to generate eps files, but I'm guessing that there may be an
easier way...

Cheers,
~Mike