I don't believe you can set a default program for "all bitmap images," but that you must set it for each type of bitmap image. You didn't mention what type of bitmap image the file you want to edit is (I am guessing .bmp), but there are numerous possibilities (.gif, .jpg, .tif, etc.). You should make sure the default for the type of image you are dealing with is set using the Windows file browser Tools > Options > File Types tab. Once this is set correctly, the file's icon should change to the picture icon for the MS Imaging application.
There is another way, however. You can always use the VBA SHELL function to force a particular application to try to open the file. For example:
Const MyFile = "C:My Documentsdoggie.bmp"
RetVal = Shell("C:WINDOWSMSIMAGING.EXE " & MyFile, 1)
I'm just guessing at the path and name of the MS Imaging executable--you will have to find out where it is and put the correct string in its place. The trailing blank in the string is required!