Brightness and Contrast

You can manually adjust the brightness and contrast settings, which might advisable. It's in the same area: Filters > Adjust Color > Brightness/Contrast

The Brightness/Contrast dialogue should pop up. Slide the levels around and keep checking your image to see if you like the results:

You can really go overboard with the Brightness and Contrast levels if you like:

Hey, don't foget to save the file with a new, meaningful name: myImage_CrazyContrast