popup window to show the image file on which the mouse hovers, in the space available left (right, above or below) of the Windows Explorer (with the thumbs or list view). Perhaps in conjunction with auto window raise (to make (Explorer) window active (MouseActivate), and inactive again if mouse leaves the window).
Also, the thumbs in the Windows (XP) Explorer still appear a bit small, of which Microsoft decided.
CutAway can do it, but it would be nice to switch off the left pane when Explorer shows the image previewer ("Filmstrip View"). At this point the left pane can't be switched off unless the window is made narrow enough, and thus can get in the way.