You can actually change the default BMP icon to a thumbnail version of the actual BMP file. To do this simply go to:
In the right pane change the value of default to %1.
To restore back to the normal change the vale of default back to: C:Progra~1Access~1MSPAINT.EXE,1.
Please note however that this will slow down the display rate in explorer if there are too many BMP thumbnails to display. You can use other icons too, simply enter the pathname.