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.
Caption indicators: If this is checked, then a little blue "i" indicator will appear on the upper-right corner of the thumbnail if the picture has a caption.
Published indicators: When checked, the thumbnail will display a globe indicator in the upper-left if the picture is published.
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: