I've had my Rebel for about a year now. And I've noticed dust on pictures several times, but then it goes away, and I just assumed it was loose dust or dirt that landed on top of the lens and that wind kept blowing it away. Recently I discovered more of a pattern. When I turn the aperture up, I get dust. When I have the aperture wide open, I don't. It's the stock lens, btw, which is a EFS 18-55mm. I've taken the lens off and inspected the inside of the camera and lens as best I can (which is not all that great since I'm pretty new to digital slr), and I'm not seeing anything. I have a couple of new lenses on the way, but I still want to be able to use this lens. Is it the lens, or the camera? How should I go about cleaning it? Thanks for helping a digital SLR newb. Here's two pictures taken at the smallest and largest aperture so that you can see what I'm talking about.
http://img116.imageshack.us/img116/328/img3397zh8.jpg
http://img380.imageshack.us/img380/5998/img3398ix0.jpg