[CustomEditor(typeof(TestCustom))]
public class TonScriptEditor : Editor
{
    public override void OnInspectorGUI()
    {
        TestCustom script = (TestCustom)target;

        EditorGUI.BeginChangeCheck();

        script.mySprite = (Sprite)EditorGUILayout.ObjectField(
            script.mySprite,
            typeof(Sprite),
            false,
            GUILayout.Width(128),
            GUILayout.Height(128)
        );

        if (EditorGUI.EndChangeCheck())
            EditorUtility.SetDirty(script);
    }
}