public class MyImage : Xamarin.Forms.Image
{
    public static BindableProperty OnClickProperty =
        BindableProperty.Create("OnClick", typeof(Command), typeof(MyImage));

    public Command OnClick
    {
        get { return (Command)GetValue(OnClickProperty); }
        set { SetValue(OnClickProperty, value); }
    }

    public MyImage()
    {
        GestureRecognizers.Add(new TapGestureRecognizer() {Command = new Command(DisTap)});
    }

    private void DisTap(object sender)
    {
        if (OnClick != null)
        {
            OnClick.Execute(sender);
        }
    }

}

<local:MyImage Source="{Binding Img}" OnClick="{Binding ImgTapCommand}" />

<AbsoluteLayout>
            <Image Source="clock.png" AbsoluteLayout.LayoutBounds="0.2,0.2,35,35" AbsoluteLayout.LayoutFlags="PositionProportional"/>
            <Button AbsoluteLayout.LayoutBounds="0.2,0.2,35,35" AbsoluteLayout.LayoutFlags="PositionProportional" BorderColor="Transparent" BackgroundColor="Transparent" Command="{Binding AlertMeCommand}"/>
</AbsoluteLayout>