Feature image *and* an icon for each post?

I’m new to Ghost and wanted to check I’m not somehow missing something obvious while building a custom theme.

Our use case is that for a set of posts, we would like to have a feature image to use as a banner, but in a particular collection view to use a simpler icon/illustration for each post.

I was initially looking for ways to extend the Post object to include an additional field, but that looks like it’s not possible. We can think of a couple approaches that involve some combination of css in the theme and extensive use of tags. But I wanted check here if there is a more Ghost-obvious approach to something like this that we are simply overlooking.