To make your Library content more compelling and easier to find, you might consider setting thumbnails for your Documents. This will allow Users to get a better idea of the content before viewing/downloading. 

Where are Document thumbnails displayed?

Thumbnails are visible in the Small and Large Tile Views of the Library. Users can switch between the List, Small Tile or Large Tile Views by selecting the icons in the right corner of the Library (circled in red in the image below). Thumbnails are not be visible in the List View. You can set the default layout of a Library in the Library Settings.

How do I set a Document's thumbnail? 

When you add a Document to the Library, the default thumbnail is automatically selected. (Thumbnails are not available for Folders.) For certain file types like images and PDFs, an additional thumbnail image is automatically generated. You can choose to use the system or generated thumbnail or upload your own image. 

If you want to upload your own thumbnail image you can do so by clicking the Upload File button. The image you upload should be saved as JPG, GIF or PNG and be at least 256 pixels in both dimensions.

How do I update a thumbnail?

You can add or update existing thumbnails by hovering your mouse over the Document you want to update and clicking the downward facing arrow. You'll then select Edit and update the Thumbnail field as shown above.