I was looking to test the roseta theme with the plan to purchase if it works out to get the plus features but when I changed the header image it just stayed with the stock image. Is this part of the free version that I can’t change that stuff? I’ll give it a try again later.
The header image does change from the Header Image setting.
But what you probably also want to change is the landing page static image. For that go to the Customizer > Landing Page > Slider. There you will be able to change that image or replace it with a slider.