Plugin and using wp_is_mobile

Are there some simple settings in the plugin or modification so that I can use the wp_is_mobile function to avoid loading large images when the site is being visited from a mobile device. Using wp_is_mobile() is not functioning at this point at all – mobile users are getting some pretty heavy images thrown at them. Thanks for any advice.

https://www.ads-software.com/plugins/w3-total-cache/