Another (embarrassing) CSS Question

I apologize right off the bat for asking this question because it's a CSS
question and I'm not quite sure it's really that related to docbook. But
here I go anyway.

As usual, my problem focuses around spacing. I've inserted a graphic using
the docbook <graphic> tag, and what I want to do is, using a CSS, I want to
control the spacing above and below the graphic (i.e. top and bottom
margin). So in my CSS I tried referring to div.img, which it just did not
like.

Does anyone have any ideas on how I can control this spacing? (I also tried
out of curiosity the <literallayout> tag, but I think that's only for
text.)

Thanks for your patience.

JB