Many, many kinds. A typical photo is quite sparse. Oh sure, there is a lot of variation in the individual pixels etc., but it's usually a picture of something with shape and fairly strongly defined visual characteristics (which is what made it interesting enough to record in the first place). Random chroma noise, by contrast, is not sparse at all. In that sense, it has a much higher information content than a picture.
That Terry Tao blog linked above is by far the best entry point I've found into the subject but I'm really not qualified to explain or simplify it well, I'm afraid.
That Terry Tao blog linked above is by far the best entry point I've found into the subject but I'm really not qualified to explain or simplify it well, I'm afraid.