When a media file is uploaded, Media Manager will automatically generate a thumbnail for the media. This image can be cropped or adjusted by the admin by dragging the window to encompass the desired view.
If the admin wishes to have a different image as the thumbnail for the media, the ideal image dimensions are 697 x 465.
On iPad (non-retina) they are 239 x 205.
The maximum size for thumbnails is 2MB.