RewriteEngine on  
RewriteCond %{HTTP_REFERER} ^$ [OR]  
RewriteCond %{HTTP_REFERER} !^http://site/.com/.*$ [NC] 
RewriteCond %{HTTP_REFERER} !^http://www/.site/.com/.*$ [NC]  
RewriteRule .*.([jpg|gif|jpe?g]+)$ - [NC,F]

ExpiresActive On 
ExpiresByType image/gif "access 1 week" 
ExpiresByType image/jpg "access 1 week" 
ExpiresByType image/jpeg "access 1 week" 
ExpiresByType image/png "access 1 week" 
ExpiresByType gif "access 1 week" 
ExpiresByType jpg "access 1 week"