You can customize the look of your thumbnails by using the Customize Thumbnails Options from the "View" main menu (or by using the thumbnail size dropdown in the upper right).
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You can change the appearance of them by using the following options.
Auto Size: This controls whether thumbnails will be a fixed size or will automatically be sized according to the number of thumbnails visible. If Auto Size is checked, then thumbnails will shrink or grow to make the best use of the display space (within the range of the Min. and Max. sizes). If this is unchecked, then the thumbnails will always be the size specified by the Height and Width settings.
Min. and Max Size: These options (only visible if Auto Size is checked) change the minimum and maximum size the thumbnails will appear when being auto-sized.
Height and Width: These options (only visible if Auto Size is unchecked) change the height and width of the currently selected thumbnail size. It does not change the storage of the thumbnail - it merely changes the appearance.
Border: This option changes the amount of border around each thumbnail.
Background Color: This controls the background color of each thumbnail.
Icon indicators -- These icons are displayed beneath the thumbnail (and beneath any of the text chosen below). They indicate different characteristics that an image may have.
Captions exist: If this is checked, then an icon representing a caption (cloud) will appear on all images that have captions.
Published: When checked, the thumbnail will display a globe indicator if the picture is published (public or private)
Video: When checked, the thumbnail will display a video camera icon for all videos.
Archive Existing: If this is checked, a CD icon will appear for any images that have been archived (at least once) and are still exist on your system.
Archive Deleted: If this is checked, a CD icon with a red "X" will appear any images that have been archived and do not exist on your system.
Display below thumbnails -- This section controls what is displayed beneath each thumbnail.
Filename: If checked, displays the filename beneath the thumbnail.
Filesize: If checked, displays the filesize beneath the thumbnail.
Dimensions: If checked, displays the picture dimensions and bits/pixel beneath the thumbnail.
Picture Date: If checked, displays the picture date beneath the thumbnail. If the picture does not have a date, then it does not display anything for the date.
3D Effects: If checked, displays the thumbnail with additional shadowing and has an edge around each thumbnail.
The following thumbnail shows an example with every option turned on: