If you click the edit icon during the upload process, it will open the folder where the thumbnails are stored. You can then open the file in an image editing software, (Paint, Photoshop, etc..)
Once your done editing just save the file. The preview picture might not update, going back into the level and then going through the upload process again will make the preview update for you.

Or if you want another image, the conditions are : Same name and extension. (MyMachine.png)
Be warned, if you save your machine, your image will be overwritten.

Uhm, question also means, how to change it in existing workshop item.