Try to addopt this:
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^(.*?)example\.com(/.*)?$ [NC]
RewriteRule ^images/products/.+/(.+).png$ images/low-res/$1.png [L]
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^(.*?)example\.com(/.*)?$ [NC]
RewriteRule ^images/products/.+/(.+).png$ images/low-res/$1.png [L]
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^(.*?)example\.com(/.*)?$ [NC]
RewriteRule ^downloads/brochures/.+/(.+).jpg$ images/low-res/$1.jpg [L]
This rules looking for if the request comes from your site or directly from a third party. And rewrites then the request.