Adicionar imagens que percorram a tela

olá, gostaria de uma ajuda para saber como adicionar imagens que "fiquem descendo a tela" constantemente de tempo em tempo com o objetivo de criar obstáculos com elas.

#include <stdio.h>
#include <allegro5/allegro.h>
#include <allegro5/allegro_image.h>
#include <allegro5/allegro_acodec.h>
#include <allegro5/allegro_audio.h>

const float FPS = 60;
const int SCREEN_W = 640;
const int SCREEN_H = 480;
const int AVIAO_SIZE = 80;

enum MYKEYS { KEY_UP, KEY_DOWN, KEY_LEFT, KEY_RIGHT, KEY_SPACE };

int main(int argc, char **argv)
{

 ALLEGRO_DISPLAY *display = NULL;
 ALLEGRO_EVENT_QUEUE *event_queue = NULL;
 ALLEGRO_TIMER *timer = NULL;
 ALLEGRO_BITMAP *aviao_principal = NULL;
 ALLEGRO_SAMPLE *sample=NULL;//Audio
 float bouncer_x = SCREEN_W/2 - AVIAO_SIZE / 2.0;
 float bouncer_y = SCREEN_H-90 - AVIAO_SIZE / 2.0;
 bool key[4] = { false, false, false, false };
 bool redraw = true;
 bool doexit = false;

 if(!al_init()) {
 fprintf(stderr, "failed to initialize allegro!\n");
 return -1;
 }

 if(!al_install_keyboard()) {
 fprintf(stderr, "failed to initialize the keyboard!\n");
 return -1;
 }

 timer = al_create_timer(1.0 / FPS);
 if(!timer) {
 fprintf(stderr, "failed to create timer!\n");
 return -1;
 }
 if(!al_install_audio()){
 fprintf(stderr, "failed to initialize audio!\n");
 return -1;
 }

 if(!al_init_acodec_addon()){
 fprintf(stderr, "failed to initialize audio codecs!\n");
 return -1;
 }

 if (!al_reserve_samples(1)){
 fprintf(stderr, "failed to reserve samples!\n");
 return -1;
 }

 sample = al_load_sample("audio/somambiente.wav");
 if (!sample)
 {
 fprintf(stderr, "Falha ao carregar sample.\n");
 al_destroy_display(display);
 return false;
 }

 display = al_create_display(SCREEN_W, SCREEN_H);
 if(!display) {
 fprintf(stderr, "failed to create display!\n");
 al_destroy_timer(timer);
 return -1;
 }

 al_init_image_addon();
 aviao_principal = al_load_bitmap("imagens/aviaoprincipal.png");
 if(!aviao_principal) {
 fprintf(stderr, "failed to create avio_principal bitmap!\n");
 al_destroy_display(display);
 al_destroy_timer(timer);
 return -1;
 }

 al_set_target_bitmap(aviao_principal);

 // al_clear_to_color(al_map_rgb(255, 0, 255));

 al_set_target_bitmap(al_get_backbuffer(display));

 event_queue = al_create_event_queue();
 if(!event_queue) {
 fprintf(stderr, "failed to create event_queue!\n");
 al_destroy_bitmap(aviao_principal);
 al_destroy_display(display);
 al_destroy_timer(timer);
 return -1;
 }

 al_register_event_source(event_queue, al_get_display_event_source(display));

 al_register_event_source(event_queue, al_get_timer_event_source(timer));

 al_register_event_source(event_queue, al_get_keyboard_event_source());

 al_clear_to_color(al_map_rgb(0,0,0));

 al_flip_display();

 al_start_timer(timer);

 while(!doexit)
 {
 ALLEGRO_EVENT ev;
 al_wait_for_event(event_queue, &ev);
 al_play_sample(sample, 1.0, 0.0,1.0,ALLEGRO_PLAYMODE_LOOP,NULL);

 if(ev.type == ALLEGRO_EVENT_TIMER) {
 if(key[KEY_UP] && bouncer_y >= SCREEN_H/2) {
 aviao_principal=al_load_bitmap("imagens/aviaoup.png");
 bouncer_y -= 4.0;
 }

 if(key[KEY_DOWN] && bouncer_y <= SCREEN_H - AVIAO_SIZE - 50) {
 aviao_principal=al_load_bitmap("imagens/aviaoprincipal.png");
 bouncer_y += 4.0;
 }

 if(key[KEY_LEFT] && bouncer_x >= 7.0) {
 aviao_principal=al_load_bitmap("imagens/aviaoleft.png");
 bouncer_x -= 4.0;
 }

 if(key[KEY_RIGHT] && bouncer_x <= SCREEN_W - AVIAO_SIZE - 7) {
 aviao_principal=al_load_bitmap("imagens/aviaoright.png");
 bouncer_x += 4.0;
 }

 redraw = true;
 }
 else if(ev.type == ALLEGRO_EVENT_DISPLAY_CLOSE) {
 break;
 }
 else if(ev.type == ALLEGRO_EVENT_KEY_DOWN) {
 switch(ev.keyboard.keycode) {
 case ALLEGRO_KEY_UP:
 key[KEY_UP] = true;
 break;

 case ALLEGRO_KEY_DOWN:
 key[KEY_DOWN] = true;
 break;

 case ALLEGRO_KEY_LEFT:
 key[KEY_LEFT] = true;
 break;

 case ALLEGRO_KEY_RIGHT:
 key[KEY_RIGHT] = true;
 break;
 }
 }
 else if(ev.type == ALLEGRO_EVENT_KEY_UP) {
 switch(ev.keyboard.keycode) {
 case ALLEGRO_KEY_UP:
 aviao_principal=al_load_bitmap("imagens/aviaoprincipal.png");
 key[KEY_UP] = false;
 break;

 case ALLEGRO_KEY_DOWN:
 aviao_principal=al_load_bitmap("imagens/aviaoprincipal.png");
 key[KEY_DOWN] = false;
 break;

 case ALLEGRO_KEY_LEFT:
 aviao_principal=al_load_bitmap("imagens/aviaoprincipal.png");
 key[KEY_LEFT] = false;
 break;

 case ALLEGRO_KEY_RIGHT:
 aviao_principal=al_load_bitmap("imagens/aviaoprincipal.png");
 key[KEY_RIGHT] = false;
 break;

 case ALLEGRO_KEY_ESCAPE:
 doexit = true;
 break;
 }
 }

 if(redraw && al_is_event_queue_empty(event_queue)) {
 redraw = false;

 al_clear_to_color(al_map_rgb(135,206,250));

 al_draw_bitmap(aviao_principal, bouncer_x, bouncer_y, 0);

 al_flip_display();
 }
 }

 al_destroy_bitmap(aviao_principal);
 al_destroy_timer(timer);
 al_destroy_display(display);
 al_destroy_event_queue(event_queue);
 al_destroy_sample(sample);

 return 0;
}