Could that even occur in nature?
Pretty much no. Anything much larger than Saturn will start to experience enough self-induced gravitational force that it'll collapse down into denser matter without growing physically larger. Jupiter is significantly more massive than Saturn, but only slightly larger in diameter, as illustrated in
http://xkcd.com/681/ For there to be a nebula in between the pov and the camera in that shot, it would have to be a small nebula, as from the destruction of a large fleet or space station orbiting the planet in question. I'd suggest it could be formed at the planets' L1 trojan point, but the angles of the solar illumination would seem to belie that possibility - you'd only see something eclipsing the planet from an L1 point when the planets disc was fully illuminated - unless the nebula was formed from some sort of destroyed craft in a wide orbit outside the plane of the ecliptic around the L1 point, and even then...
The only practical way for a cloud of nebular gas to exist in front of a planet that takes up that large a portion of the FOV (assuming the FOV is higher than ~5 degrees of arc or so, which is not a big leap to take) is for something to have blown up there recently enough that its gaseous debris field has not yet been significantly degraded by the various magnetic and gravatic forces that tend to surround planets.
