1. An image synthesis processing apparatus comprising: an extraction unit configured to extract from a background image a background image portion which is located in a synthesis object region; a first image rotation processing unit configured to rotate the background image portion by an instructed rotation angle in an opposite direction which is opposite to an instructed rotation direction; a synthesis processing unit configured to synthesize the background image portion after rotation and an original foreground image including replacement color pixels which are replaced by pixels in the rotated background image portion, thereby generating a synthesis image; a second image rotation processing unit configured to rotate the synthesis image by the instructed rotation angle in the instructed rotation direction; and a write-back unit configured to write back the synthesis image rotated by said second image rotation processing unit to a whole of the background image.