Loader#spritesheet()

Adds a Sprite Sheet to the current load queue.

The file is not loaded immediately after calling this method. The file is added to the queue ready to be loaded when the loader starts.

To clarify the terminology that Phaser uses: A Sprite Sheet is an image containing frames, usually of an animation, that are all equal
dimensions and often in sequence. For example if the frame size is 32x32 then every frame in the sprite sheet will be that size.
Sometimes (outside of Phaser) the term "sprite sheet" is used to refer to a texture atlas.
A Texture Atlas works by packing together images as best it can, using whatever frame sizes it likes, often with cropping and trimming
the frames in the process. Software such as Texture Packer, Flash CC or Shoebox all generate texture atlases, not sprite sheets.
If you've got an atlas then use Loader.atlas instead.

The key must be a unique String. It is used to add the image to the Phaser.Cache upon successful load.

Retrieve the file via Cache.getImage(key). Sprite sheets, being image based, live in the same Cache as all other Images.

The URL can be relative or absolute. If the URL is relative the Loader.baseURL and Loader.path values will be prepended to it.

If the URL isn't specified the Loader will take the key and create a filename from that. For example if the key is "alien"
and no URL is given then the Loader will set the URL to be "alien.png". It will always add .png as the extension.
If you do not desire this action then provide a URL.