I don't think Org syntax provides any way to do this at present, but
ignore the rest of this message if anyone knows better.
Possible workarounds:
1. Wrap your figure in a #+BEGIN_ADDITIONALCLASS...#+END_ADDITIONALCLASS
block, to wrap the div.figure in a div.additionalclass. Not exactly what
you wanted, and a pain in the neck if you have many such figures, but
gives you enough to work with in CSS.
2. Add javascript to move the extra class attribute from the img to
the containing div.
3. Write an export filter.