How to add a 3rd rotation to this

I have a script that can make a UI Image rotate at 90 and -90 degrees along with the Horizontal Axis. I want to make it also rotate to 0 and 180 on the same axis on the Vertical Axis so that it can rotate 360. Here's the script...

Code (CSharp):

1.     using UnityEngine;
2.     using UnityEngine.UI;
3.     using System.Collections;
4.  
5.     public class MenuArrowH : MonoBehaviour
6.     {
7.         public float normalAngle = 180;
8.         public float leanAngle = 90;
9.         public float smooth = 100;
10.         public Image imageToLerp;
11.  
12.         // Use this for initialization
13.         void Start()
14.         {
15.  
16.         }
17.  
18.         // Update is called once per frame
19.         void Update()
20.         {
21.             Vector3 euler = imageToLerp.transform.eulerAngles;
22.             float hInput = Input.GetAxis("Horizontal");
23.  
24.             if (hInput < 0)
25.             {
26.                 euler.z = Mathf.LerpAngle(euler.z, leanAngle, Time.deltaTime * smooth);
27.             }
28.             else if (hInput > 0)
29.             {
30.                 euler.z = Mathf.LerpAngle(euler.z, -leanAngle, Time.deltaTime * smooth);
31.             }
32.             else
33.             {
34.                 euler.z = Mathf.LerpAngle(euler.z, normalAngle, Time.deltaTime * smooth);
35.             }
36.  
37.             imageToLerp.transform.eulerAngles = euler;
38.         }
39.     }

How could I do this? I've tried with a lot of issues. What can I do?