Open the following Registry key -->

HKEY_Classes_Root\Paint.Picture\DefaultIcon

The default setting for this key is: C:\Progra~1\Access~1\MSPAINT.EXE, 1

This tells Windows to use the generic Paint icon. Change this setting to %1. Each BMP file uses it's own built in icon handler then. Go to Explorer and select View, Refresh or press F5 to update the display....

Note, if you have installed another program that has taken over the bmp extension, then you will need to modify the default icon for that program. To find out what program owns the extension, expand the HKEY_Classes_Root key and find the .bmp key. Highlight that key and see what the value is for (Default) in the right hand pane. Normally the value would be "Paint.Picture". If yours is different, you will need to change the default icon for that type under HKCR, instead of Paint.Picture.