Item.cs

using System.Collections;
using System.Collections.Generic;
using System.IO;
using UnityEngine;
#if UNITY_EDITOR
using UnityEditor;
#endif

public class Item : MonoBehaviour
{
    public string itemName;
    [ContextMenuItem("set image", "SetImage")]
    public Texture2D itemImage;
#if UNITY_EDITOR
    protected void SetImage()
    {
        Texture2D texture2D = AssetPreview.GetAssetPreview(gameObject);
        var bytes = texture2D.EncodeToPNG();
        File.WriteAllBytes(Path.Combine(Directory.GetCurrentDirectory(), "Assets", "ItemImage", gameObject.name) + ".png",bytes);
        AssetDatabase.Refresh();
        Debug.Log(Path.Combine(Directory.GetCurrentDirectory(),"Assets","ItemImage"));
        itemImage = AssetDatabase.LoadAssetAtPath(Path.Combine( "Assets","ItemImage", gameObject.name) +".png", typeof(Texture2D)) as Texture2D;
    }
#endif
}