The thumbnail displayed in the list view while on the Manage applications, My applications, or Review applications pages will always be the first image attachment as defined by the attachment order in the application itself.
If a particular image should be displayed as the thumbnail, ensure that image is ordered first on the Attachments tab in the application. Image order can be easily changed by the applicant or grants 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 application, no image will be displayed.
Note that for the voting review mode, if no image file is attached to the application, the thumbnail displays like this: