Normalized ngrx entities id reference - ngrx

Question

I am trying to normalise my API data with normalizr.
Basically my RESTful API provides a /items resource with all CRUD operations and a /items/:item-id/pictures sub resource with CRUD operations as well.
GET /items returns something a deep nested response that needs normalization:
[
{
id: "item-id-1",
name: "Item One",
pictures: [
{
id: "picture-id-1",
mimeType: "image/png"
},
{
id: "picture-id-2",
mimeType: "image/jpeg"
}
]
},
{
id: "item-id-1",
name: "Item One",
pictures: []
}
]
Now for my question: Should I normalise the code with a reference of all picture ID on the item entity or a reference to the item on the picture entity?
Reference on the item:
{
items: {
"item-id-1": {
id: "item-id-1",
name: "Item One",
pictures: ["picture-id-1", "picture-id-2"]
}
...
},
pictures: {
"picture-id-1": {
id: "picture-id-1",
mimeType: "image/png"
},
"picture-id-2": {
id: "picture-id-2",
mimeType: "image/jpeg"
}
...
}
}
vs. reference on the picture:
{
items: {
"item-id-1": {
id: "item-id-1",
name: "Item One",
}
...
},
pictures: {
"picture-id-1": {
id: "picture-id-1",
itemId: "item-id-1"
mimeType: "image/png",
},
"picture-id-2": {
id: "picture-id-2",
itemId: "item-id-1"
mimeType: "image/jpeg"
}
...
}
}
My application will provide actions to delete and add single pictures to an item. With the reference on the item I feel like it is much more expensive if I just delete an image with DELETE /items/:item-id/pictures/:picture-id. Then I need to have an item reducer function that loops through every pictures array in the item store and update the state there as well as in the pictures store. But all the examples for redux and normalizr use this approach
The reference on the picture I think is much more straight forward but I do not know how to achieve that result with normalizr. (I could write it myself pretty easily tho)