I want to use an image at the top for every page on this blog http://www.RozKayInc.com
The tutorials/videos give steps for more complex designs. But I’d just like to have a header image w/ navigation menu. Similar to this other website http://www.wendyg.com
Are we talking about the home page or sing post page?
For a static page, you can build it as you wish, I’m not sure what your difficulty is.
For a sing post page, you can activate featured image option at customizer > layout > blog > featured image, add a featured image to the post in post editor, then the featured image will show automatically.
If you can describe the issue you are having, we should be able to help more.