Access property of an array with objects

I have the following two arrays:

<?php

$array = array (
  0 => '{"id":1,"revisions_id":1,"name":"Bitcoin","symbol":"BTC","image":"bitcoin.png","sector":"sdfsf","country_of_origin":"Country of Origin","created_at":"2018-01-03 15:49:23","updated_at":"2018-01-03 15:49:23","revision_status":1}',
  1 => '[{"id":1400,"instruments_id":1,"revisions_id":1,"firstname":"adfasdf","lastname":"asdfasd","twitterAccount":"fasdfasdf","created_at":"2018-01-12 20:32:49","updated_at":"2018-01-12 20:32:49","name":"Bitcoin","symbol":"BTC","image":"bitcoin.png","sector":"sdfsf","country_of_origin":"Country of Origin","revision_status":1},{"id":1400,"instruments_id":1,"revisions_id":1,"firstname":"Sathosi","lastname":"Nakomoto","twitterAccount":"www.twitter.com\\/Sathoshi","created_at":"2018-01-12 20:32:49","updated_at":"2018-01-12 20:32:49","name":"Bitcoin","symbol":"BTC","image":"bitcoin.png","sector":"sdfsf","country_of_origin":"Country of Origin","revision_status":1},{"id":1401,"instruments_id":1,"revisions_id":1,"firstname":"adfasdf","lastname":"asdfadsf","twitterAccount":"sadfadsf","created_at":"2018-01-12 20:57:05","updated_at":"2018-01-12 20:57:05","name":"Bitcoin","symbol":"BTC","image":"bitcoin.png","sector":"sdfsf","country_of_origin":"Country of Origin","revision_status":1},{"id":1401,"instruments_id":1,"revisions_id":1,"firstname":"Sathosi","lastname":"Nakomoto","twitterAccount":"www.twitter.com\\/Sathoshi","created_at":"2018-01-12 20:57:05","updated_at":"2018-01-12 20:57:05","name":"Bitcoin","symbol":"BTC","image":"bitcoin.png","sector":"sdfsf","country_of_origin":"Country of Origin","revision_status":1},{"id":1401,"instruments_id":1,"revisions_id":1,"firstname":"Sathosi","lastname":"Nakomoto","twitterAccount":"www.twitter.com\\/Sathoshi","created_at":"2018-01-12 20:57:05","updated_at":"2018-01-12 20:57:05","name":"Bitcoin","symbol":"BTC","image":"bitcoin.png","sector":"sdfsf","country_of_origin":"Country of Origin","revision_status":1}]',
);

print_r("Get Image" . "\n");
print_r($array['0'][1]['image'] . "\n");
print_r("#################" . "\n");
print_r("Get Firstname of first sub-array" . "\n");
print_r($array['1'][0]['firstname'] . "\n");

As you can see I would like to access the image property of the first array and firstname of the second array within the first sub-array.

Any suggestions what I am doing wrong?

I appreciate your replies!