$mw = MainWindow->new;
$images{severity1} = $mw->Photo(-file=>"sev1.xpm", -format=>'xpm');
$images{severity2} = $mw->Photo(-file=>"sev2.xpm", -format=>'xpm');
$images{severity3} = $mw->Photo(-file=>"sev3.xpm", -format=>'xpm');
$severity_button = $mw->Button(
     -image => $images{severity1},  # The default severity
     -width=>16, 
     -height=>16, 
     -command => \&CallNotepad);
...
sub ChangeToSeverity2 {
     $severity_button->configure(-image => $images{severity2});
}