The thumbnail displayed in the list view while on the Manage entries, My entries, or Judge entries pages will always be the first image attachment as defined by the attachment order in the entry itself.
If a particular image should be displayed as the thumbnail, ensure that image is ordered first on the Attachments tab in the entry. Image order can be easily changed by the entrant or awards manager by clicking the image number in the top left corner of the image field:
The first image will be the one displayed:
Thumbnails are images only, no other document types will be displayed. If there are no image attachments in the entry, no image will be displayed.
Note that for the voting judging mode, if no image file is attached to the entry, the thumbnail displays like this: