Instant ThumbView Description:

Instant ThumbView is a tiny image viewer that displays image thumbnails in a tooltip window, when mouse pointer is hovered over an image file in Windows Explorer. It supports a lot of popular image file formats, including BMP, GIF, JPEG, PNG, WMF, EMF, TIFF and is very handy for quick image viewing.
To bring it up, open any folder with image files in Windows Explorer, and then hover an image file with your mouse pointer.
Instant ThumbView displays the thumbnail of selected image file in a popup window just next to the mouse pointer.
To adjust thumbnail dimensions, set displaying options and select image file formats use its Configuration Manager.