function move_pic(mv){
	if (pid+mv+2<pic_ary.length&&pid+mv+1>0){
		switch (mv){
			case 1:
			for (i=0;i<3;i++){
				obj=return_by_id('pic_'+i);
				if (i!=2) {
					obj2=return_by_id('pic_'+(i+1));
					obj.src=obj2.src;
				}
				else obj.src=pic_ary[pid+3];
			}
			break;
			case -1:
			for (i=3;i>0;i--){
				obj=return_by_id('pic_'+(i-1));
				if (i!=1) {
					obj2=return_by_id('pic_'+(i-2));
					obj.src=obj2.src;
				}
				else obj.src=pic_ary[pid-1];
			}
			break;
		}
		pid+=mv;
	}
}