Then you should ask the question right.
So the problem is not how to get the image but how to use the function or better the returned type.
GetPixelsPtr() returns the pointer to the first element of the pixel array that represents the image, it has the size GetWidth() x GetHeight() x 4 as stated in the
documentation.
Now you can itereate over it and do what ever you want with the pixel information.
Additionally it seems like you're using SFML 1.6. I adivce you to use SFML 2, because 1.6 is waaay outdated.
(Btw @Laurent if you're reading this, the doc in SFML 2 also says 'GetWidth() * GetHeight() * 4' although it's now 'getSize().x x getSize().y x 4')