It looks like you're kind of making a painting app; like MS Paint, etc.
In that case, you need an ALLEGRO_BITMAP that acts as a canvas for the user to draw to. So, let's get started.
Add this towards the top of main, as another variable:
ALLEGRO_BITMAP *canvas = NULL;
Add this during initialization, probably right after creating the cursor, sand, bouncer bitmaps:
canvas = al_create_bitmap(SCREEN_W, SCREEN_H);
This creates a canvas the size of the screen.
Add this after the code for clearing bouncer and cursor:
al_set_target_bitmap(canvas); al_clear_to_color(al_map_rgb(0,0,0));
That clears the canvas to black.
Inside the code where you call drop_sand do this instead:
al_set_target_bitmap(canvas); drop_sand(cursor_x, cursor_y); al_set_target_bitmap(al_get_backbuffer(display)); redraw = true;
This will draw the sand to the canvas.
Now in the code where you redraw the screen (line 271 in the code you posted), modify to this:
al_set_target_bitmap(al_get_backbuffer(display)); al_draw_bitmap(canvas, 0, 0, 0); // Draw the canvas over the entire screen draw_cursor(cursor_x, cursor_y); // Draw the cursor on top (on the screen) draw_ball(bouncer_x, bouncer_y);
That will "refresh" the screen with the current canvas, and draw the cursor and ball.
Right now, you're using the screen as your canvas. These new modifications just create an actual bitmap for the canvas, so you don't overwrite it every time 