Ok, quickly looking at the mess of a system they are running in this software, it looks like a bad combination of templating, php and xml. The good news is that system runs a language database for the most part, by the looks of it. Every page still must physically load from a file though, whether a php page dragging multiple templates and languages from the DB to create the page or whether just a basic page, all pages are an actual formed object.

With any PHP or HTML type page, you can drop code see more directly into the page to affect the layout. The main root of the install contains all the major pages in PHP. If you want a global banner, I would suggest header or footer, as this CMS does not come easily for page management by the looks of it, unlike say drupal or the like.

If you look in /templates you will find both /base & / tmpl_uni folders in which contain the page information depending on what you are asking the system to do. Looking at it, as I haven't modified this CMS just yet, you could adjust them within those callout templates. I would honestly stick with just header and footer banners though for hard coding modification.

The admin obviously contains the page builder system... which looks ok for basic text oriented tasks and moving the already included blocks around, though having a quick play with it, you can actually create HTML blocks in pages by dragging the "HTML Blocks" block from the samples location at the bottom of the pages, into the page, then click it to insert your own custom HTML into a page.... seems they did think about how to insert banners and customising after all with ease.

You can simply create your own pages if you like through the system and implement all custom HTML for your entire site if you like.

Admin > Builders > Pages Builder - Click page you want to insert > Bottom of page drag "HTML Block" into page > Then click that block once inserted and modify. Simply keep dragging the block from samples to your page each time you want to create custom HTML, ie. insert a banner.

Upload the banner to your images directory, insert HTML to code in the banner.