Why do none of the commands in the IMAGE tool bar work?

I have converted a batch of MS PowerPoint presentations to LibreOffice Impress. They are mostly pictures. Now I cannot edit any of the images, eg: using Crop merely resizes the images without removing any of the pieces I wish to cut out.
I cannot change the Contrast or Brightness of any image - nothing happens if I use these commands.
Am I over-looking an important step?