Diagram keeps handle to background image

I create a BackgroundBrush for Binding on Diagram.Background property.

ImageSource imageSource = (ImageSource)new ImageSourceConverter().ConvertFromString(@"C:\Users\Public\Pictures\Sample Pictures\clara.jpg");
if (imageSource != null)
    BackgroundBrush = new ImageBrush(imageSource);

If I look with ProcessExplorer to the handles I can see that my app is keeping the handle to the image (clara.jpg).

I use many other images in node templates but there is no handle to these images.

Is this a problem in diagram or in the way I make or use the brush?