What's happening is that your LCD brightness is set too high. Therefore images that are dark look ok on the LCD and images that are fine are washed out. Try going into the setup menu and turning down the brightness on the LCD.
You shouldn't trust the LCD to tell you how your pictures look either. There is a graph called a histogram that shows the amount of brightness from black to white, and only that will tell you whether the picture is really too dark or washed out.
You can find out more about histograms here: