If you just want a static banner image on your home page (this is not the same as a header image, which appears at the very top of every page), you can go to Appearance>Widgets on your WordPress admin menu. From there, you would want to place an image in the “Banner” widget position. Recommended dimensions for that image are 2560×450 pixels.
The easiest way I’ve found to insert an image into a widget area is by installing the “Black Studio TinyMCE Widget” plugin. You will get a new “Visual Editor” widget that you can use on your Widgets page. Insert that widget into the Banner widget position, then click on the “Add Media” button to insert your image into the banner area. The TinyMCE widget is a real must-have!