This 'How to:' article explains how to add preview images for your skins that you either created on your own or have not been added with the help of the install feature.
It makes only sense to do this with skins that don't get changed though.
The benefit of it is not only a preview image, but also that your skin doesn't get lost anymore.
Skins that haven't been installed and will be deleted from your skins list, will usually be removed from ALL journals that have used these before. So if you delete a skin by accident it could happen that you end up with a lot of blank journals, all formatting is lost and the content might be a mess as well