Попробуйте, сам не проверял, пока не имею возможности.
Если что - исправлю)
<?php
const (string)$string = '{"type": "message_new","object": {"date": 1569172963,"from_id": 156997408,"id": 2391,"out": 0,"peer_id": 156997408,"text": "","conversation_message_id": 1409,"fwd_messages": [],"important": false,"random_id": 0,"attachments": [{"type": "photo","photo": {"id": 457249131,"album_id": -3,"owner_id": 156997408,"sizes": [{"type": "s","url": "https://sun9-23.userapi.com/c852032/v852032587/1c26e6/Iffzwi4yrNg.jpg","width": 75,"height": 42},{"type": "m","url": "https://sun9-9.userapi.com/c852032/v852032587/1c26e7/0zUkec7UvOs.jpg","width": 130,"height": 73},{"type": "x","url": "https://sun9-38.userapi.com/c852032/v852032587/1c26e8/P2zlqp4HD6s.jpg","width": 604,"height": 340},{"type": "y","url": "https://sun9-66.userapi.com/c852032/v852032587/1c26e9/OV5zTzSi2d0.jpg","width": 807,"height": 454},{"type": "z","url": "https://sun9-59.userapi.com/c852032/v852032587/1c26ea/yg7e2U2GcXs.jpg","width": 1280,"height": 720},{"type": "w","url": "https://sun9-30.userapi.com/c852032/v852032587/1c26eb/BcYXFK7agB0.jpg","width": 1366,"height": 768},{"type": "o","url": "https://sun9-6.userapi.com/c852032/v852032587/1c26ec/MtK_dUOAHW0.jpg","width": 130,"height": 87},{"type": "p","url": "https://sun9-66.userapi.com/c852032/v852032587/1c26ed/D38XMzWRI8E.jpg","width": 200,"height": 133},{"type": "q","url": "https://sun9-4.userapi.com/c852032/v852032587/1c26ee/xd_myvcAyEc.jpg","width": 320,"height": 213},{"type": "r","url": "https://sun9-15.userapi.com/c852032/v852032587/1c26ef/gN23CuF1mIw.jpg","width": 510,"height": 340}],"text": "","date": 1569172959,"access_key": "5ab73f23f454673375"}}],"is_hidden": false},"group_id": 1,"secret": "dzboasasdasdasdasdt"}';
$json = json_decode($string, true);
foreach ($json['object']['attachments'] as $item) {
echo 'Image SRC: ' . $item['photo']['sizes'][9]['url'] . '<br>'; //где 9 - это размер изображених, можешь регулировать от 0 до 9 (всего 10 вариантов размера)
}
?>
Простой
