When you look at the contents of a folder in Explorer, files with a known open association display a mini-thumbnail icon of the associated program.

For bitmaps (.BMP), you can cause Explorer to display the actual bitmap. Use Regedt32 to browse to HKEY_CLASSES_ROOT\.bmp.

In the right hand pane, you will find a that contains an openCommandString.

Navigate to HKEY_CLASSES_ROOT\openCommandString\DefaultIcon

Change the string value to %1. Logoff/Logon and display a folder that contains bitmaps. If your Explorer View is not set to Large Icon, temporarily switch to that View. See tip 497 if you wish to permantently increase the size of this thumbnail.