What you are looking for might be the Page Layout. When you create a new page in your application or would like to change an existing one, you can change that part on top.
The following image shows a new page that just has been created.
Double click on the greyed out part on top – you will see the following edit popup:
Here you find the Layout – you can select a different layout. That isthe first thing you need to know.
The second one is that it is possible to change that layout or even to create your own ones.
You can add an image to the layout by adding it here. Every page that uses that layout will also show that image.
So far the rough outline, it is also well documented https://docs.mendix.com/refguide/layout#1-introduction here