You need to edit .htaccess file and add following line right after current lines. You can edit the file from cpanel or FTP. Also there is a WordPress Plugin that allows to edit .htaccess file.
Remember it’s a very advanced thing to do. So make sure to backup current file before editing. Any error will cause site not loading.
/* Prevent image hotlinking in WordPress */
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?yourwebsite.com [NC]
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?google.com [NC]
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?facebook.com [NC]
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?twitter.com [NC]
RewriteRule \.(jpg|jpeg|png|gif)$ - [F]
This will let only youtube, google, facebook and twitter to directly Link you images. All other websites will be blocked.