In some cases, owners of other websites can use images from your website by copying direct link to images on their websites. But when a visitor sees your images on these websites, he uses resources and traffic of your hosting.
Step 1. Open.htaccess file
Step 2. Make the following changes:
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www.)?site.com/ [nc]
RewriteRule .*.(gif|jpg|png)$ http://site.com/close.gif[nc]
Note! Instead of site.com write your domain name and create close.gif image. This image will shown instead of the pasted one.