Fix handling not existent images with ImageMagick

Execution dies if ImageMagick can't load an image: let's return false if
we can't load/process the image (plus let's free GD handles).
Fix PHPDoc

Let's describe better the create method...
Fix PHPDoc

State in the docs that the create method will only downsize the original image
Merge pull request #1672 from mlocati/fix-handling-bad-images

Fix handling not existent images with ImageMagick