SELECT producto.id_producto, producto.nombre, producto.id_imagen,
       imagen.id_producto_imagen, imagen.url_imagen

       FROM tabla_productos producto JOIN tabla_imagenes imagen
       ON producto.id_imagen = imagen.id_producto_imagen;

Array
    (
        [id_producto] => 1
        [0] => 1
        [nombre] => Iphone 6
        [1] => Iphone 6
        [id_imagen] => 1
        [2] => 1
        [id_producto_imagen] => 1
        [3] => 1
        [url_imagen] => P001.jpg
        [4] => P001.jpg
    )

    Array
    (
        [id_producto] => 1
        [0] => 1
        [nombre] => Iphone 6
        [1] => Iphone 6
        [id_imagen] => 1
        [2] => 1
        [id_producto_imagen] => 1
        [3] => 1
        [url_imagen] => P001_1.jpg
        [4] => P001_1.jpg
    )

    Array
    (
        [id_producto] => 1
        [0] => 1
        [nombre] => Iphone 6
        [1] => Iphone 6
        [id_imagen] => 1
        [2] => 1
        [id_producto_imagen] => 1
        [3] => 1
        [url_imagen] => P001_2.jpg
        [4] => P001_2.jpg
    )

FROM tabla_productos producto JOIN tabla_imagenes imagen
       ON producto.id_producto = imagen.id_producto_imagen;