To reproduce:

1. Start MITK workbench and load an image
2. Right-click on the data node in the data manager to open the context menu
3. Move the Opacity slider to the left / right and see how the brightness / intensity is decreased / increased but the image is not getting more opaque / transparent

or

1. Start MITK workbench and load an image
2. Open the properties view
3. Change the opacity-property and see how the brightness / intensity is decreased / increased but the image is not getting more opaque / transparent

This happens since a140adda resp. 9f89c812