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)".
Kama wewe huna tovuti, kuongeza picha unataka kutumia na tovuti ambapo kuhifadhi picha (mfano, Instagram, Flickr, google gari), kufanya sanamu hadharani kupatikana na kuingia anwani picha ndani ya boksi na "Desturi Image (URL)".