In particular, it is important that the 'preview.jpg' is NOT called 'preview.jpg', but has a unique [to you] title, such as 'preview1.jpg' 'preview2.jpg' 'screenshot-tuesday.jpg'....that sort of thing.
If you are uploading a lot of, say, logons, and you name them the same, then all the different files will have the identical preview image instead of their own.
Not terribly clever, you'll admit...
