The easiest way is to just copy an image and then click paste image. You don't need the url. If you have a mouse, you can do it by right clicking. On a tablet, you need to press and long-hold the image. You will then see the 'copy image' menu that you need to select. Next, click on your KidzTalk post and select paste by doing the same thing you did to copy (right click or long hold).