That's great news Michel - I had no idea Gimp existed outside of Ubuntu. So there it is Ray, download Gimp.
I usually go to:
and then select curves or levels. With either, you just slide the various levels or curves up or down making the image darker, brighter, more contrast, less, etc., until you find that the details are more accentuated. Sometimes I get into changing or manipulating the colors but usually these two ways work fine.