That’s the best thing about MODx, we have a resource called TV (Template Variables), which allow us to assign TV on specific template, and that template will have that same TV placement on every pages. After having this TV, you can assign default content on it, so that it will have the same content all over the page with the same template, and later when you want to have different content for this, you can just change it on each pages and assign an undefault value, and that changes will only affect the specific pages.
If you want images and etc, you can see it on TV, they have a a lot of variety in input type for TV, and they even have a binding option to allow you to fetch data from database and etc, and if it’s not enough for you, they have a display system called widget, which allow you to have a special formating output for your TV.
For more information, you can check out the documentation.
http://www.modxcms.com/documentation
Welcome to MODx community.
Sincerely,