public readonly PGrid Pg1;
...
    Pg1 = new PGrid();
...
    public class PGrid : Grid
    {
        public static readonly DependencyProperty PGridLockProperty = DependencyProperty.Register("IsLocked", typeof(Boolean), typeof(PGrid));
        public Boolean IsLocked
        {
            get { return (Boolean)GetValue(PGridLockProperty); }
            set { SetValue(PGridLockProperty, value); }
        }

                                <Button x:Name="LockButton" Click="Lock_Click" Style ="{DynamicResource RoundCorner2}" Width="36">
                                    <StackPanel>
                                        <!--<Image Source="Pictures/security_lock.png" ToolTip="Save" Width="28"/>-->
                                        <Image Width="28">
                                            <Image.Style>
                                                <Style TargetType="{x:Type Image}" >
                                                    <Setter Property="Source" Value="Pictures/security_lock.png" />
                                                    <Style.Triggers>
                                                        <DataTrigger Binding="{Binding Pg1.IsLocked}" Value="False">
                                                            <Setter Property="Source" Value="Pictures/security_unlock.png" />
                                                        </DataTrigger>
                                                    </Style.Triggers>
                                                </Style>
                                            </Image.Style>
                                        </Image>
                                    </StackPanel>
                                </Button>