Code

<diag:Diagram
	x:Name="diagram"
	AllowDrop="True"
	DragOver="OnDragOver"
	Drop="OnDrop"
	....

void OnDragOver(object sender, DragEventArgs e)
{
	e.Handled = true;
	e.Effects = DragDropEffects.None;

	var formats = e.Data.GetFormats();
	if (Array.IndexOf(formats, DataFormats.FileDrop) != -1)
	{
		var fileNames = (string[])e.Data.GetData("FileName");
		foreach (string file in fileNames)
		{
			if (file.ToLower().EndsWith(".png") ||
				file.ToLower().EndsWith(".jpg"))
			{
				e.Effects = DragDropEffects.Copy;
				Mouse.OverrideCursor = Cursors.Hand;
				break;
			}
		}
	}
}

void OnDrop(object sender, DragEventArgs e)
{
	e.Handled = true;
	e.Effects = DragDropEffects.None;

	var formats = e.Data.GetFormats();
	if (Array.IndexOf(formats, DataFormats.FileDrop) != -1)
	{
		e.Effects = DragDropEffects.Copy;
		Mouse.OverrideCursor = null;

		var position = e.GetPosition(diagram.DocumentPlane);
		var fileNames = (string[])e.Data.GetData(DataFormats.FileDrop);
		foreach (string file in fileNames)
		{
			if (file.ToLower().EndsWith(".png") ||
				file.ToLower().EndsWith(".jpg"))
			{
				var rect = new Rect(position, new Size(150, 150));
				var image = new Image
				{
					Source = Utilities.LoadImage(file)
				};
				var nodeAdapter = new DiagramNodeAdapter(image)
				{
					Bounds = rect
				};
				diagram.Nodes.Add(nodeAdapter);

				position.Offset(40, 40);
			}
		}
	}
}