Home page hero image sizing is fixed

The banner image items in the home page hero image carousel use a fixed width of 500 pixels:

<img src="{{ theme.homepageImageBannerB.thumbnail(500, 'auto')}}" alt="demo_slide_1" />

This causes any uploaded images for the theme settings to only partially fill the header horizontally, even if they're high resolution.

For some reason the default images are displayed at full width. Not sure what is causing this but uploaded images should behave more predictably and automatically size to use the full width of the div.