If you don't have a website, add the image you want to use to a website where you store images (e.g., instagram, flickr, google drive), make the image publicly accessible and enter the image address into the box by the "Custom Image (URL)".
Kung wala kang isang website, magdagdag ng mga imahe na nais mong gamitin sa isang website kung saan ka mag-imbak imahe (e.g., instagram, Flickr, google biyahe), gumawa ng imahe ang access ng publiko ang at ipasok ang address ng larawan sa kahon ng "Pasadyang Imahe (URL)".