"Sometimes Millennium chooses to "customize" its display in Explorer all on its own, refusing to display thumbnail pictures in its display. This may be due to a corrupted line in the Registry.
Find out by going through Start, Run, and typing REGSVR32 THUMBVW.DLL without quotes in the Run field. This updates the Registry by inserting the THUMBVW.DLL file to allow viewing of .BMP and .JPG files."
Source - click here