Define phpdoc for magic methods on Image class.

Providing phpDoc for magic methods makes development much more easy as the IDE can provide auto completion. :-)
![auto_completion](https://cloud.githubusercontent.com/assets/343404/4678684/71b5f648-55f9-11e4-8f24-5c866b6afa08.png)

PS: It must be a one-liner otherwise phpdocumentor will not parse the description :-\