I run a few web sites. I don't care if people copy any images off them to use on their own site. I don't mind if they use one of my own images to link back to my site. I do care if they just use an image on their site because they are to lazy to copy the image or to save thair bandwidth.

A friend suggested an Apache re-write rule to prevent it happening. After a tweak, I've found it most useful, I just need to wait long enough to take effect.

RewriteEngine on
RewriteCond     %{REQUEST_FILENAME} \.(gif|jpe?g|png)$       [NC]
RewriteCond     %{HTTP_REFERER}     !^$
RewriteCond     %{HTTP_REFERER}     !(www\.)?domain\.com     [NC]
RewriteCond     %{HTTP_REFERER}     !google\.                [NC]
RewriteCond     %{HTTP_REFERER}     !search\?q=cache         [NC]
RewriteRule     (.*)                -                    [F,NC,L]

UPDATE: Note that domain.com means your domain.