This shot was a last minute thing and they wanted to have the pictures taken there. The camera was on full auto and I just hit the shutter. I just feel the camera should output a better cleaner image. It seems like a hardware problem to me.
An example of what I want from my 20D is here. Bobby G shot the images with a 20D. This got me thinking about what the problem could be.
A photograph is made up of a number of elements. The photographer can, if he chooses control most of these. If you are shooting JPG with your 20D, then you are leaving many of the choices ultimately to the camera. You can set sharpness, contrast and color and also White balance. If you choose the optimums for a given scene, and you pose the scene correctly and you choose the correct exposure, then you'll get the type of photos you mention BobbyG taking. But keep in mind that he did take control of his scenes. He posed them correctly and the used light modifiers to get what he wanted. You basically took a snapshot. If I had shot the same prom scene you did with the background that you've used, I would have posed it differently and exposed it differently. I would have moved the girl in yellow closer to the rest of the girls and made the group tighter. I would have moved them to the right some to get rid of the bar running through the girl in yellow's hair. I would have then framed it to zoom in on the girls more. Now without seeing the rest of the room, I can't say how I would have tackled the best possible shot in this situation. I would have pumped the contrast a bit, and changed the color balance. I would have tried to hide the poles behind the girls and then probably cloned out the poles that would show up behind one or two of them. Lot's of possibilities.
But to really analyse what you want to accomplish, you need to tell us specifically what you don't like about the photo. If you want more contrast, you can add that in the camera. If you want better WB, you can change that in the camera. If you think the exposure is off, you can control that. If you think the lighting is uneven, you can change that. And so forth.
But you started off by talking about a dirty sensor and yet the photo does not have problems that I can see relative to a dirty sensor.