{
  "postTitle" : "Test post",
  "images" : [
              {
               "url" : "http:...",
               "title": "First image"
              },
              {
               "url" : "http:...",
               "title": "First image"
              }
             ]
}

[postMapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:@"images" toKeyPath:@"images" withMapping:postImagesMapping]];