optimizing select ... not in (select ...)

I have this query :

select distinct on (Pictures.PictureID) * from Pictures where Pictures.PictureID not in (select distinct PictureID from
Keywords);

and I find it a bit slow. Does anybody have suggestions to run this
faster ? (I have indexes on PictureID on both Pictures and Keywords) 

-- 
Laurent Martelli
laurent@aopsys.com              http://www.bearteam.org/~laurent/