featured post slide

Hi, Im getting crazy trying to fix a couple of images for the home page as featured post slider changing the ones you are using as default. I imagine Im doing something wrong as Im not able to fix it. If I add the pictures ID nothing happen. Shall I create a post that include a featured image and add his ID ? How I should title it ? how shall I post it (public ? Private ?) Can you give me some instruction please. Many thanks Enrico