Insert still image as existing video background

hi, I want to add a still image as video background.Not as overlay on video,it is easy to do it,but as a background behind the existing video.I know it is available to set backgroundColor in AVMutableVideoComposition,apple document says 'patterns and other color refs that are not supported will be ignored '.Anyone have solution to solve this?It is difficult in AVMutableComposition level to solve this,if anyone can solve this by ffmpeg \ GPUImage or some other methods,please let me know.