Yep .htaccess works great... Try using this
ErrorDocument 404 /index.html
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://www.yourdomain.com [NC]
RewriteCond %{HTTP_REFERER} !^http://yourdomain.com [NC]
RewriteRule .*\.(gif|GIF|jpg|JPG|mpg|MPG|Mpeg|mpeg|MPEG|rm|RM) $
http://www.yourdomain.com [R,L]
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Logic
It works most of the time