<img src="link"> (quotation marks included, change link to your desired URL)
You sadly can't use images from just any website, they have to be from an image-hosting site; I know Imgur is popular but I've had a lot of trouble getting the images to load, so I'd actually recommend using catbox.moe! The good thing about Catbox is that you can make an account and sort your images into files.
Typing just <img src="link"> will get you a square image, though you can use CSS to change properties about it like corner rounding and making it move across the screen. Hope this helps!