Hi,
In my opinion, there isn't such a thing as the best place to put images, but for most of the sites I work on/have worked on (either umbraco or not), I keep the layout images as close to the css as possible.
/css
/css/img/...
Just my 2 cents.
Regards,
/Dirk
level 1 & 2 certified - umbraco MVP 2008/2009 - umbraco blog at
netaddicts.be - working on an integrated forum4umbraco