Customize SEO Image of password-protected files or folders

Hello, 

I embed through links various files I need to share with my clients. Some of my links lead to folders, and others, to mere single files. All are password-protected because I don't want clients who aren't concerned to be able to access them and I also want to help combat copyright violations. Mind you, I do understand that this would seemingly defeat the purpose of protecting via password since the entire point of said preference is that the contents be protected from view. However, some image still has to be shown for links that are embedded in a different site and that lead to password-protected content. Said image, for the moment, is simply the Dropbox icon. 

What I would like is to be able to choose an image that previews my file or folder. I'd like to be able to choose an image that showcases that folder's theme or the company of the resources therein, for example, even when the file or folder is password-protected. As far as I can tell, this option is simply not available.

Can someone kindly explain how to achieve such, or someone at Dropbox offer a solution? If you were reading a book, can you imagine only seeing the name of the inventor of italics instead of the font itself as you read over some music title or book title or something that is being said? It would ruin the effect of that font for the reader, and your experience would be ruined too. It's happened to all of us on computers where some code is carried over by accident and imposed on the reader of some text. Effectively, this is what happens when you embed links in your site that show an SEO image of Dropbox instead of an image that you feel would reflect that folder's contents or at least the file itself if singular. 

Thank you kindly for your help, sadly though I doubt that anything will be able to be done as this does seem like a minor detail that not many people deal with.