Why the background image quality is reduced?
I use this code:

al_draw_scaled_bitmap(picture, 0, 0, pw, ph, 0, 0, 1280, 800, 0);

The image has a good quality when displayed with a viwer.

Different scaling methods. The default is a Point/Nearest Neighbor scale. To use something similar to what an image viewer would use, do this:

ALLEGRO_STATE state;
al_store_state(&state, ALLEGRO_STATE_NEW_BITMAP_PARAMETERS);
al_set_new_bitmap_flags(ALLEGRO_MIN_LINEAR | ALLEGRO_MAG_LINEAR);
al_load_bitmap...
al_restore_state(&state);