Comments by Holger Boskugel (Top 1 by date)

I would use existing Drag&Drop functionality of WPF (http://msdn.microsoft.com/de-de/library/ms742859.aspx). For use of this span an empty label over the whole grid (Grid.ColumnSpan="5" Grid.RowSpan="5") while grid is not able to handle Drag&Drop.

<Label Grid.ColumnSpan="5" Grid.RowSpan="5"/>

Give the grid AllowDrop = True. On Drop event calculate position based on grid and place your image at the related field in grid.