Leonid, I am afraid that Spaces has too high of a learning curve for some of our users. Me, for example.

Yes, to upload an additional preview file (with no processing necessary) is exactly what we need to do! Can you please explain how to add this new preview image field? And also tell us where these preview files will be stored.

What size of a image file would you recommend? That is, the height and width in pixels. Landscape or portrait?

On our site, only moderators can upload files which have been approved. Therefore uploading an extra preview file is no problem at all. 

Thank you for considering this matter, Leonid. I believe others may also be interested in your simple solution. As time goes by.