Extend Unity's Image editor with custom inspector GUI

CustomImageEditor.cs

using UnityEditor;
using UnityEditor.UI;
using UnityEngine;
using UnityEngine.UI;

[CustomEditor(typeof(Image))]
public class CustomImageEditor : ImageEditor
{
    public override void OnInspectorGUI()
    {
        GUILayout.Button("my button");

        base.OnInspectorGUI();
    }
}