I'm afraid that's not how it works. Unlike your avatar, you will have to find an image hosting site (e.g: Imgur, Photobucket, etc) and upload your images there. Afterward, you can copy the URL of the image and put it un brackets like Zeldamaster12 showed you. Keep in mind, image URLs and image post URLs are not the same, unless the URL ends on ".gif" or any other image format, what you're copy-pasting is not an image URL.
Oh my god this image made my day XDD