STEP:

var dir = point_direction(x, y, mouse_x, mouse_y)

switch ((((dir + 22.5) mod 360) + 360) mod 360) div 45 {
    case 0:
        sprite_index = right;
        break;
    case 1:
        sprite_index = tr;
        break;
    case 2:
        sprite_index = up;
        break;
    case 3:
        sprite_index = tl;
        break;
    case 4:
        sprite_index = left;
        break;
    case 5:
        sprite_index = bl;
        break;
    case 6:
        sprite_index = down;
        break;
    case 7:
        sprite_index = br;
        break;
}