Hi johanna,
You didn’t need to edit to add banner images in pages. To add banner images, you need to add Text widget on Banner Wide Widget position. And on the text widget content area you can add html tag for images as shown below:
If you want to display banner images in specific pages then you can use Jetpack widget visibility plugin. See for reference :How to show/hide widgets on pages?
If this didn’t clearify your doubt then please let us know, we are here to help you.
Thanks