Improve consistency of banner image aspect ratio on different screen sizes

[SophieAmnesty]

Feedback from the user survey:

It can be very difficult to get the banner image the right size, and just seems to be a case of trial and error.

This respondent works on the Security Lab site - you can see on their home page they have an illustrated banner image which was quite difficult for them to make work as a hero image without it being cropped in weird ways.

In these kinds of cases, it can get tricky because the aspect ratio changes on different screen sizes.