To maintain the banner proportions on a mobile device, there are a couple of key things that need to be done.
First of all, the banner image dimension needs to be the exact size desired for the full site. If the banner needs to cover the full width of your site, the image should be at least 1200-1500px wide.
Secondly, to ensure you maintain the same banner proportion, it is best to use Contain for the banner size. Here's how to do this:
- Go to Style > Designer.
- Under the Color section, click Banner Image.
- For the Size option, select Contain.
- Click Save.