The mitk::DataNode header includes the header of mitk::Image (via mitkImageSource.h).
Although this is not a real problem, it is unnecessary and increases compile times when working with mitk::Image (because many classes depend on DataNode).
The DataNode is supposed to have a dependency on BaseData, not the Image.
Unfortunately this dependency has been there quite a long time now. Thus removing the include will result many other classes to miss the "mitkImage.h" because the transitive include has been removed.
It's not hard to do, just takes some time to make the code compile again...