How to edit banner image on login page

Go to this page in the theme editor:
https://joannaczech.myshopify.com/admin/themes/124707012631/editor?previewPath=%2Faccount%2Flogin

You will see you can add a desktop image and a mobile image

To select / change an image, click the dropdown and select "change"

The checkbox setting for stretching images to full width allows it to fit full width on screens. If not checked, it will be smaller width wide on screen sizes.