One of the basic premise of the Paperflite platform is that it allows you maximum maneuverability. We'd like to enable you do as you like! That's why we constantly build features with 'you' in mind. :)
Now, if you want to change your document thumbnail, here's how you can do it easily:
Log in to Paperflite and click on the asset whose thumbnail you'd like to change. Notice the option to edit at the top of the asset's menu bar.
Click on it and you'll notice that the option to change your thumbnail gets activated. On the asset thumbnail, you can click on "Upload New Picture."
Clicking on it allows you to use a picture from your local computer and upload. Once you're done, hit 'Update' at the bottom to save your changes.
You can follow the same process to edit the thumbnail that you've previously uploaded too.