Congratulations on building your first WordPress theme. You still have some to go, though. As you can read in the Codex the normal way to add a header is not by simply placing an <img> tag in the header. WP allows you to upload a header image in the backoffice which you can access with
<img src="https://wordpress.stackexchange.com/questions/261543/<?php header_image(); ?>">
If you want different header images on different pages of your theme, you can use conditional tags to build it yourself or you can use a plugin like WP Display Header.
As you can see in the source code of your site, there are different classes on the <body> tag. These you can use in your style.css to assign different background colors, for instance for home and single pages.
body.home {background:red;}
body.single {background:blue;}
Hopefully this will help you take the next step. Be sure to read the tips and tricks in the Codex.