You wanna hit the insert image button on the message keys headboard below the title box and have the image adress to add. For example
Hit this button. Load up a image address, from internet images or if you want something from your PC to show up, you need to host the image on a site like imgur or imgbb. Right click on the image you want to post, choose copy image adress or copy image url, paste it into the insert image box which pops up after selecting the button above and boom. Or if you do [IMG] [IMG] insert the address in between the brackets to get a pic to show up.