You can add images using the image <img> tag. Inside the img tag is a the source attribute, written as "src". The source of the image is the location of the image. This can either be the url of the image if its stored online or the file path of the image if its stored on your computer. Notice the url must be enclosed with quotation marks. You can change the size of an an image by adjusting its width. Also notice that the img tag is rather unique in that id doesn't require a closing tag. The width of an image can be set as number of pixels(px) or percent. Play around with the image by entering your own image url and adjusting the width of the image.
You can adjust the height of an image as well, but generally not a good idea because it can distort the image. When you adjust the width, the height automatically adjusts as well to maintain the correct proportion for the image. You don't need a closing tag for the img tag because all of the information for the image is contained within the opening tag. There is no information that comes after the opening tag so there is no need for a closing tag. A good resource for finding beautiful, free to use images is Unsplash