Okay so I think I found a few CSS tweaks to make the banner image somewhat more predictable and nicer looking:
```css
background-size: cover; /* was auto before, which means it's displayed in original size. now it's scaled up/down that it just fills the space */
background-repeat: no-repeat; /* was repeat before, ugly and has no effect with "background-size: cover" anymore */
background-position: center; /* was 0 0 before. center has a better chance to keep the important part of the image */
```
Now I only need to find out the aspect ratio so we can recommend it on the edit page.