I used to think it could take more to get a subfolder trusted than say an individual file and I guess this sways me to use files on most websites I created (back in the day). Once subfolders are trusted, it’s 6 or half a dozen, what the actual difference is in terms of ranking in Google – usually, rankings in Google are more determined by how RELEVANT or REPUTABLE a page is to a query.
QUOTE: “If you have comments on your site, and you just let them run wild, you don’t moderate them, they’re filled with spammers or with people who are kind of just abusing each other for no good reason, then that’s something that might kind of pull down the overall quality of your website where users when they go to those pages might say, well, there’s some good content on top here, but this whole bottom part of the page, this is really trash. I don’t want to be involved with the website that actively encourages this kind of behavior or that actively promotes this kind of content. And that’s something where we might see that on a site level, as well.” John Mueller, Google 2016
Another reason is that if you're using an image as a link, the alt text for that image will be treated similarly to the anchor text of a text link. However, we don't recommend using too many images for links in your site's navigation when text links could serve the same purpose. Lastly, optimizing your image filenames and alt text makes it easier for image search projects like Google Image Search to better understand your images.