Brightness
Adjust the slider to the right to make the image brighter, adjust to the left to make it darker.
Contrast
Adjust the slider to the right to increase contrast, and to the left to decrease contrast.
Brightness and contrast are adjusted for each pixel in the image so this operation may take a bit of time for large images.
Tip: tune the Internal Rendering DPI (User panel) down before adjusting brightness/contrast on large photos. A lower value there (e.g. 70) will speed up the adjustments. Later on you can always tune it up again. No image information gets lost, because the original remains in the document file.