Binding ImageSource from String

I would like to bind string form my custom dependency onject  to my button style template.

In my Dictionary.xaml

<Style x:Key="{x:Type local:myButton}" TargetType="{x:Type local:myButton}"><Setter Property="Template"><Setter.Value><ControlTemplate TargetType="{x:Type local:myButton}"><Border x:Name="Border" CornerRadius="2" BorderThickness="0" Padding="5"><Image Name="image" Source="{Binding ImageSource, RelativeSource={RelativeSource FindAncestor,AncestorType={x:Type local:myButton}}}"></Image></Border></ControlTemplate></Setter.Value></Setter></Style>

Class with my dependency property

        public static readonly DependencyProperty ImageSourceProperty = DependencyProperty.Register("ImageSource", typeof(string),
typeof(myButton), new PropertyMetadata());

        public string ImageSource
        {
            get { return (string)GetValue(ImageSourceProperty); }
            set { SetValue(ImageSourceProperty, value); }
        }

In User Control, it works when ImageSource is as follow

<my1:myButton ImageSource="/ApplicationName;component/Images/buttonImage.png" Content="Button Text" x:Name="myButton1"  />

I would like to have it much sorter, just the file name to make it works

<my1:myButton ImageSource="buttonImage.png" Content="Button Text" x:Name="myButton1"  />