ALLEGRO_BITMAP
https://www.allegro.cc/manual/5/ALLEGRO_BITMAP

typedef struct ALLEGRO_BITMAP ALLEGRO_BITMAP;

It's a struct which contains information about the bitmap you loaded, things like width, height etc.

With contemporary graphics hardware, it is best to arrange all animation frames in a single bitmap. This is called an "atlas bitmap". You can then either use al_draw_bitmap_region to draw the frames, or al_create_sub_bitmap to create individual bitmaps for the frames efficienly.

And a happy new year to you to!

To add to beoran's advice. You should also set up a timer so that every time the timer fires off, you update the animation (switch to the next image). You will have to decide for yourself how often you want the time to fire off.

In my Deluxe Pacman 2 game I have one large PNG image for my Pacman character in a 5x5 grid of images that make up the animation. When you are moving, the first 5 images are used and when you die, it goes through the rest.

My Allegro 5 code to load in my main pacman image, with this 5x5 grid of images is...

I then later set up a timer for the pacman animation...

   // Create a timer for updating pacman movement
   // once InitPacman is done
   pacman.timer = al_create_timer(1.0f / pacman.ts);
   if(!pacman.timer) {
      // Handle errors here
   }
   al_register_event_source(event_queue, al_get_timer_event_source(pacman.timer));
   al_start_timer(pacman.timer);

pacman.ts holding the speed I wish to update this.

And in my game loop event checker, I check for the timer and update pacman's animation when it fires...

   case ALLEGRO_EVENT_TIMER:
      // PACMAN TIMER
      if(event.timer.source == pacman.timer) {
         // Do pacman stuff here
      }

From the Allegro wiki (https://www.allegro.cc/manual/5/al_lock_bitmap) on using al_lock_bitmap() with ALLEGRO_LOCK_READONLY as I do in my code.

“ALLEGRO_LOCK_READONLY - The locked region will not be written to. This can be faster if the bitmap is a video texture, as it can be discarded after the lock instead of uploaded back to the card.”

When you read/write a bitmap, Allegro has to update the bitmap after you are done, which takes time. If you lock it as read only, that means you won't be altering it and Allegro can simply discard it once you're done without wasting time updating it. Which makes sense.