WPF using MVVM: DataBinding with RelativeSource

I have a control and within that control I have a resource with a data tempalte:

  <DataTemplate DataType="{x:Type local:FlowModel}">
    <Image Source="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type vm:MainViewModel}}, Path=MainViewModel.ImagePath}"/>
  </DataTemplate>

 xmlns:vm="clr-namespace:CortexMonitoringTool.ViewModel"

I have vm set to my ViewModel folder, I am implementing mvvm. I cannot get my binding to work and I am unsure why not.

Can some tell me if my relative binding is correct, if it can actually see my property 'ImagePath' in my MainViewModel class?

public String ImagePath
    {
        get
        {
            return _imagePath;
        }
        set
        {
            if (_imagePath == value)
            {
                return;
            }
            _imagePath = value;
            RaisePropertyChanged("ImagePath");
        }
    }

Thank you.