You need to provide an output port (not an output data) as input.
Connecting ITK images to VTK pipine directly is not commonly done because it is rare that you start a new complex application from scratch and you need to deal with such low level details.
Also note that since VTK does not support oriented images (yet), if you simply connected ITK filters directly to a VTK visualization pipeline, then all non-axis aligned images would appear at incorrect position/orientation. In 3D Slicer, we use a custom class (vtkMRMLVolumeNode) that stores a vtkImageData and additional metadata (such as image orientation) and orientation is injected into the visualization pipeline at the actor level.