The layer disposal method defines the way each the displayed image is to be modified after the current 'frame' of an animation has finished being displayed (after its 'delay' period), but before the next frame on an animation is to be overlaid onto the display.
Value for $dispose:
Undefined: 0 -> No disposal specified (equivalent to 'none').
None: 1 -> Do not dispose, just overlay next frame image.
Background: 2 -> Clear the frame area with the background color.
Previous: 3 -> Clear to the image prior to this frames overlay.