When using the Save Image tool in OpenBuildings Designer the resulting image is lighter and almost seems a bit "faded". In addition, many of the edges are not very smooth.
In order to correct the brightness of the resulting image, try changing the Gamma Correction value from the default 1.70 to 1.00:
In order to help smooth the edges choose an Antialising Quality value other than the default None. In this example High has been used: