Unity: Saving/Loading Sprites as Json

I was following a tutorial where I need to save the sprite that belongs to an object. I am creating a database of items and would of course want the correct image to come with the item. This is how I built my Item database:

ItemObject

[System.Serializable]
public class ItemObject{

    public int id;
    public string title;
    public int value;
    public string toolTip;
    public bool stackable;
    public string category;
    public string slug;
    public Sprite sprite;


    public ItemObject(int id, string title, int value, string toolTip, bool stackable, string category, string slug)
    {
        this.id = id;
        this.title = title;
        this.value = value;
        this.toolTip = toolTip;
        this.stackable = stackable;
        this.category = category;
        this.slug = slug;
        sprite = Resources.Load<Sprite>("items/" + slug);
    }

ItemData

[System.Serializable]
public class ItemData {

    public List<ItemObject> itemList;

}

I then Save/Load ItemData.itemList. That works great. As you can see I use "slug" to load the sprite of my Item, slug is basically a code-friendly name (ex: golden_hat), then I have the same name for my Sprite.

This also works great, but Unity saves the Sprite InstanceID, which always changes on start, so if I exit Unity and load my data, it will get the wrong Image.

My Json:

{
    "itemList": [
        {
            "id": 0,
            "title": "Golden Sword",
            "value": 111,
            "toolTip": "This is a golden sword",
            "stackable": false,
            "category": "weapon",
            "slug": "golden_sword",
            "sprite": {
                "instanceID": 13238
            }
        },
        {
            "id": 1,
            "title": "Steel Gloves",
            "value": 222,
            "toolTip": "This is steel gloves",
            "stackable": true,
            "category": "weapon",
            "slug": "steel_gloves",
            "sprite": {
                "instanceID": 13342
            }
        }
    ]
}

So I'm guessing it wont work to do it this way, is there some other way to save a Sprite in Json? Or do I have to load the correct Sprite at runtime every time using my "slug"?