You put a URL in there, the URL of your image. You can use an image host such as Imgur to upload your image online. Then, get the URL of the image (e.g. by right-clicking and selecting "Copy Image Address" or similar from the menu, or by dragging the image to the address bar to navigate directly to the image, and then the address bar will have the image's URL in it), and paste that between the two IMG tags.