Do you mean so that you see a thumbnail on the folder or thumbnails for files? The latter should be created when you save but may be missing or may not work in some items you buy or download - just make a PNG file 91 pixels square and name it to match the library file  (for Poser content, foe filename.cr2 you'd call it filename.png, for D|S content you'd add the extension so for filename.dsb you'd need filename.dsb.png). To add a thumbnail to a folder, again make a 91 pixel square png, and put it in the folder that holds the folder you want a tumbnail on and give it the the name of the folder (so to put a thumbnail on ContentPeopleVictoria 4 you'd create a png, called Victoria 4.png, and place it in the ContentPeople folder).