Composite one image onto another - Справочник: PHP5.4

bool Imagick::compositeImage ( Imagick $composite_object , int $composite , int $x , int $y [, int $channel = Imagick::CHANNEL_ALL ] )

Composite one image onto another at the specified offset.

composite_object: Imagick object which holds the composite image
compose: Composite operator. See Composite Operator Constants
x: The column offset of the composited image
y: The row offset of the composited image
channel: Provide any channel constant that is valid for your channel mode. To apply to more than one channel, combine channeltype constants using bitwise operators. Refer to this list of channel constants.

В случае успешной работы возвращает TRUE.