I would never say it was a fool-proof way. But most of the 
people who steal images are either not very knowledgable or 
else not motivated enough to bother trying to find 
alternate ways to get images.  BTW, you can also use JS to 
disable the "view source" command, which adds another level 
of difficulty to the issue at hand.  I don't bother with JS 
at all, because I'm perfectly happy with my pages as it is, 
but my sister gets really upset when some kid (usually an 
aoler) steals her pictures and puts them on their own 
website without any reference and without asking 
permission.  For most of these kids, it's enough to protect 
the page with JavaScript.  But like I said, nothing can 
protect from a truly determined site-theif except to not 
put the pictures online at all.


Jennifer