Un Scriptable Object (Asset) que basicamente es una lista de Sprites, con un Editor que lo muestra como Reorderable List. Ademas que se puede usar onda "ListaDeImagenes miLista; Sprite miSprite = miLista[2]" y Anda

ListaDeImagenes.cs

/*
 * Agradecimientos a
 * Valentin Simonov https://twitter.com/valyard
 * Unity: make your lists functional with ReorderableList
 * http://va.lent.in/unity-make-your-lists-functional-with-reorderablelist/
 * 
 * 
 */

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

[CreateAssetMenu()]
public class ListaDeImagenes : ScriptableObject {

    public List<Sprite> sprites = new List<Sprite>();

    public Sprite this[int index]
    {
        get
        {
            if (sprites == null) return null;
            else if (index < 0 || index >= sprites.Count) return null;
            else return sprites[index];
        }
    }

    public int Count
    {
        get
        {
            if (sprites == null) return 0;
            else return sprites.Count;
        }
    }

    public void Add(Sprite s)
    {
        sprites.Add(s);
    }
    public void Remove(Sprite s)
    {
        sprites.Remove(s);
    }

    public static ListaDeImagenes operator +(ListaDeImagenes li, Sprite s)
    {
        li.Add(s);
        return li;
    }
    public static ListaDeImagenes operator -(ListaDeImagenes li, Sprite s)
    {
        li.Remove(s);
        return li;
    }
}

#if UNITY_EDITOR
[CustomEditor(typeof(ListaDeImagenes))]
public class ListaDeImagenesEditor : Editor
{
    ListaDeImagenes laLista;
    Vector2 scrollViewPos;
    ReorderableList reorderableList;

    private void OnEnable()
    {
        laLista = target as ListaDeImagenes;
        reorderableList = new ReorderableList(serializedObject, serializedObject.FindProperty("sprites"),true,true,true,true);
        reorderableList.drawElementBackgroundCallback = DrawElement;
        reorderableList.drawHeaderCallback = (Rect rect) => {
            EditorGUI.LabelField(rect, reorderableList.serializedProperty.displayName);
        };
    }

    public void DrawElement(Rect r, int index, bool activo, bool focused)
    {
        if (index < 0) return;
        var element = reorderableList.serializedProperty.GetArrayElementAtIndex(index);
        EditorGUI.PropertyField(new Rect(r.x+EditorGUIUtility.labelWidth,r.y,r.width - EditorGUIUtility.labelWidth, EditorGUIUtility.singleLineHeight), element,GUIContent.none);
    }

    public override void OnInspectorGUI()
    {
        serializedObject.Update();
        reorderableList.DoLayoutList();
        serializedObject.ApplyModifiedProperties();
        scrollViewPos=EditorGUILayout.BeginScrollView(scrollViewPos);
        int cantColumnas = 3;
        float anchoCosas = Mathf.FloorToInt ((EditorGUIUtility.currentViewWidth-EditorGUIUtility.singleLineHeight*3) / cantColumnas);
        for (int i=0; i<laLista.Count; i++)
        {
            if(i%cantColumnas==0)EditorGUILayout.BeginHorizontal();

                Rect r = EditorGUILayout.GetControlRect(GUILayout.Width(anchoCosas),GUILayout.Height(anchoCosas));
                r.width -= 3;
                r.height -= 3;
                Sprite s = laLista[i];
                if (s)
                {
                    EditorGUI.DrawPreviewTexture(r, s.texture);
                }

            if (i % cantColumnas == cantColumnas-1) EditorGUILayout.EndHorizontal();
        }
        if (laLista.Count%cantColumnas != 0) EditorGUILayout.EndHorizontal();
        EditorGUILayout.EndScrollView();
    }
}
#endif