Feature Images on Item show page are being styled as Thumbnails

[murny]

From #1666

We displaying this feature image as a "thumbnail" on Item show page which has the following css:

.j-thumbnail {
    max-width: 135px;
    max-height: 175px;
}

But we have more real-estate to work with here? Should we increase the size of these feature
images on the show page, so users can view the image better?

This actually appears to be a regression. We were originally styling them as feature images
before using the following css: https://github.com/ualbertalib/jupiter/blob/integration_postmigration/app/javascript/styles/image.scss#L6-L9. But when we added the fallback image about 9 months ago, we changed everything (including the feature images) to be styled as thumbnails (.j-thumbnail)