When you move the mouse over a file (thumbnail) in the thumbnail view and popup tips are enabled, the thumbnail (if present) will be displayed as an image tip (similar to a standard windows tool tip). If you have unchecked thumbnails, under the Thumbnail Display section, in Options | Preferences - Thumbnail View, the image tips will display immediately on a mouse move. Otherwise, the standard tool tip delay is used before displaying the image tip. To configure when and how popup image tips are displayed, click the Setup button under Options | Preferences - Thumbnail View.
Note: You can disable the image tip display altogether by unchecking the Enable popup tips option under Options | Preferences - Thumbnail View.