[
    {
        "id": "8488",
        "name": "Tenby",
        "area": "Area1"
    },
    {
        "id": "8489",
        "name": "Harbour",
        "area": "Area1"
    },
    {
        "id": "8490",
        "name": "Mobius",
        "area": "Area1"
    }
]

[
    {
        "id": "8488",
        "name": "Tenby",
        "area": "Area1",
        "image": "1278.jpg"
    },
    {
        "id": "8489",
        "name": "Harbour",
        "area": "Area1",
        "image": "1279.jpg"
    },
    {
        "id": "8490",
        "name": "Mobius",
        "area": "Area1",
        "image": "1280.jpg"
    }
]

<?php

$data[0]['id']="8488";
$data[0]['name']="Tenby";
$data[0]['area']="Area1";

$data[1]['id']="8489";
$data[1]['name']="Harbour";
$data[1]['area']="Area1";

$data[2]['id']="8490";
$data[2]['name']="Mobius";
$data[2]['area']="Area1";

echo json_encode($data)."<br/>";

/*Add Image element (or whatever) into the array according to your needs*/

$data[0]['image']="1278.jpg";
$data[1]['image']="1279.jpg";
$data[2]['image']="1280.jpg";

echo json_encode($data);

?>

$temp = json_decode($json);
$temp[] = new data, whatever you want to add...;
$json = json_encode($temp);

$strJSON = '[
  {
      "id": "8488",
      "name": "Tenby",
      "area": "Area1"
  },
  {
      "id": "8489",
      "name": "Harbour",
      "area": "Area1"
  },
  {
      "id": "8490",
      "name": "Mobius",
      "area": "Area1"
  }
]';

$json = json_decode( $strJSON );

$json[0]->image = "1278.jpg";
$json[1]->image = "1279.jpg";
$json[2]->image = "1280.jpg";

echo "<pre>";
print_r( $json );
echo "</pre>";

Array
(
    [0] => stdClass Object
        (
            [id] => 8488
            [name] => Tenby
            [area] => Area1
            [image] => 1278.jpg
        )

    [1] => stdClass Object
        (
            [id] => 8489
            [name] => Harbour
            [area] => Area1
            [image] => 1279.jpg
        )

    [2] => stdClass Object
        (
            [id] => 8490
            [name] => Mobius
            [area] => Area1
            [image] => 1280.jpg
        )

)