Нужно из [attachments] получить "type": "r" и выдернуть ссылку: "https://sun1-25.u...e42/IEGEFjIJAX4.jpg",

"attachments": [{
"type": "photo",
"photo": {
"id": 457303008,
"album_id": -7,
"owner_id": -62679477,
"user_id": 100,
"sizes": [{
"type": "s",
"url": "https://sun1-19.u...e3b/m8_I6vecNUE.jpg",
"width": 73,
"height": 75
}, {
"type": "m",
"url": "https://sun1-30.u...e3c/VOZ-c4IJqzs.jpg",
"width": 126,
"height": 130
}, {
"type": "x",
"url": "https://sun1-85.u...e3d/MICQY8L1lNA.jpg",
"width": 586,
"height": 604
}, {
"type": "y",
"url": "https://sun1-91.u...e3e/eSpnXUkSRRY.jpg",
"width": 750,
"height": 773
}, {
"type": "o",
"url": "https://sun1-15.u...e3f/c7f_n3I3s4s.jpg",
"width": 130,
"height": 134
}, {
"type": "p",
"url": "https://sun1-21.u...e40/ykcnGHKr6rk.jpg",
"width": 200,
"height": 206
}, {
"type": "q",
"url": "https://sun1-16.u...e41/zSIziO_ZXgY.jpg",
"width": 320,
"height": 330
}, {
"type": "r",
"url": "https://sun1-25.u...e42/IEGEFjIJAX4.jpg",
"width": 510,
"height": 526
}],
"text": "",
"date": 1579529528,
"post_id": 142046,
"access_key": "c9b92cc69d37ba3c29"
}
}],