I received a Nikon D40 for Christmas. I've just started using it, and I've noticed on every single image there is a dark... thing. I'm assuming it's a speck of dust. I don't notice the mark through the view finder, but when I look on the LCD screen (looking back on the photo's I've taken), it is always there. What is likely to be the problem? Is it simply dust? How do I get it fixed? It was a Christmas present and it was bought over a month ago, so I wouldn't be able to get an exchange in the shop. Is this permanent?