Bon, je viens de faire un essai, en fait dans ce genre de cas tu peux utiliser les outils de développeur du navigateur. Dans la console JavaScript, voici ma démarche et le résultat :
var maVarJSON = {
"resultCount":2,
"results": [
{"wrapperType":"track", "kind":"feature-movie", "trackId":354112018, "artistName":"James Cameron", "trackName":"Avatar (2009)", "trackCensoredName":"Avatar (2009)", "trackViewUrl":"https://itunes.apple.com/us/movie/avatar-2009/id354112018?uo=4", "previewUrl":"http://a1308.v.phobos.apple.com/us/r1000/073/Video/da/23/d3/mzm.cieibiha..640x478.h264lc.d2.p.m4v", "artworkUrl30":"http://a1217.phobos.apple.com/us/r1000/025/Video/be/c6/dd/mzl.chdcjxzz.30x30-50.jpg", "artworkUrl60":"http://a1281.phobos.apple.com/us/r1000/025/Video/be/c6/dd/mzl.chdcjxzz.60x60-50.jpg", "artworkUrl100":"http://a1890.phobos.apple.com/us/r1000/025/Video/be/c6/dd/mzl.chdcjxzz.100x100-75.jpg", "collectionPrice":14.99, "trackPrice":14.99, "releaseDate":"2009-12-18T08:00:00Z", "collectionExplicitness":"notExplicit", "trackExplicitness":"notExplicit", "trackTimeMillis":9702335, "country":"USA", "currency":"USD", "primaryGenreName":"Action & Adventure", "contentAdvisoryRating":"PG-13",
"longDescription":"From Academy Award®-winning director James Cameron comes Avatar, the story of an ex-Marine who finds himself thrust into hostilities on an alien planet filled with exotic life forms. As an Avatar, a human mind in an alien body, he finds himself torn between two worlds, in a desperate fight for his own survival and that of the indigenous people."},
{"wrapperType":"track", "kind":"feature-movie", "trackId":498445748, "artistName":"Lewis Schoenbrun", "trackName":"Aliens vs. Avatars", "trackCensoredName":"Aliens vs. Avatars", "trackViewUrl":"https://itunes.apple.com/us/movie/aliens-vs.-avatars/id498445748?uo=4", "previewUrl":"http://a955.v.phobos.apple.com/us/r30/Video/f5/b2/85/mzm.tlpwwejx..640x478.h264lc.d2.p.m4v", "artworkUrl30":"http://a72.phobos.apple.com/us/r30/Video/ef/5d/f5/mzi.mdnrsvev.30x30-50.jpg", "artworkUrl60":"http://a136.phobos.apple.com/us/r30/Video/ef/5d/f5/mzi.mdnrsvev.60x60-50.jpg", "artworkUrl100":"http://a553.phobos.apple.com/us/r30/Video/ef/5d/f5/mzi.mdnrsvev.100x100-75.jpg", "collectionPrice":14.99, "trackPrice":14.99, "releaseDate":"2011-09-20T07:00:00Z", "collectionExplicitness":"notExplicit", "trackExplicitness":"notExplicit", "trackTimeMillis":4792063, "country":"USA", "currency":"USD", "primaryGenreName":"Sci-Fi & Fantasy", "contentAdvisoryRating":"Unrated",
"longDescription":"A race of creatures known as the Scythe have become a plague in the universe. Cunning and powerful foes, they possess the ability to transform into anything from which it has consumed DNA. On a mission to stop the plague that her people unleashed, a beautiful alien named Ava has followed the Scythe to Earth. Meanwhile, six college friends blowing off steam on a camping trip find themselves caught up in a cat and mouse hunt with the Scythe. Not knowing what to do or who to trust, they struggle to protect themselves and keep it together. Reluctantly joining forces with Ava, they quickly learn what they are up against. Can they defeat the Scythe before it slaughters them one by one?"}]
};
Au passage, le JSON est bien valide (à priori c'était logique mais il faut toujours vérifier ses sources, par principe).
Ensuite :
console.log(maVarJSON.results[0].artworkUrl100);
J'obtiens :
http://a1890.phobos.apple.com/us/r1000/025/Video/be/c6/dd/mzl.chdcjxzz.100x100-75.jpg
C'est ce que tu voulais non ? (ou pas ?). Si tu pouvais répondre à mes questions ci-dessus ça serais cool.