The single image element is what you would use to put in a picture by itself onto your page.
Your image can come from three different sources. The Media Tab, an External Link, or you can use the Featured Image for the page. You will probably use media that has already been uploaded most of the time. You can experiment with the different image styles, however it is a good idea not to stray too far from the initial look of the page or related pages.
Before adding a picture make sure to resize it to the exact size that you want it to be on the page. As a frame of reference a page is usually 1200px wide.