I've bought a used Panasonic Lumix DMC-ZX3, but the images show a dark spot near the center. It's more visible when zoomed in (first picture below), but it's also visible otherwise. It seems to be clearer on rather dark images, while on very bright images it's not visible at all.
Where could this defect come from, and is there anything I can do? Cleaning the outside of the lens didn't help. I'm comfortable with repairing electronics, so if taking the camera apart and cleaning the inner side of the lens, I could do that too. Any advice?