How to get pixel coordinates from mouse position?

Hello everyone!
I am making an editor extension that will allow me to paint textures within editor, but I have trouble getting a pixel coordinates from a mouse click position.

I know there's some math involved, but I'm terrible at math unfortunately.. So not sure how to achieve this :/ Is there some Unity's helper method that I could use maybe? Or if there are none, what calculation is needed to get the pixel coordinates?

I would really appreciate any help! Thank you!

This is my code:

 using System;
 using UnityEditor;
 using UnityEngine;
 
 public class Painting : EditorWindow
 {
     int size = 5;
     float zoomScale = 1.0f;
 
     Rect imgRect
     {
         get
         {
             return new Rect(25, 25, this.position.width - 50, this.position.height - 50);
         }
     }
     Texture2D texture = null;
 
     [MenuItem("Painter/Open")]
     public static void OpenWindow()
     {
         var window = EditorWindow.GetWindow<Painting>("Painter");
         window.Show();
     }
 
     private void OnEnable()
     {
         this.Setup();
     }
     private void Setup()
     {
         texture = new Texture2D(Screen.width, Screen.height);
         Color32[] pixels = new Color32[texture.width * texture.height];
         for (int i = 0; i < pixels.Length; i++)
         {
             pixels[i] = new Color32(255, 255, 255, 255);
         }
         texture.SetPixels32(pixels);
         texture.Apply();
     }
 
     private void OnGUI()
     {
         Matrix4x4 oldMatrix = GUI.matrix;
 
         BeginWindows();
         Matrix4x4 Translation = Matrix4x4.TRS(new Vector3(Screen.width / 2, Screen.height / 2, 0), Quaternion.identity, Vector3.one);
         Matrix4x4 Scale = Matrix4x4.Scale(new Vector3(zoomScale, zoomScale, 1.0f));
 
         GUI.matrix = Translation * Scale * Translation.inverse;
 
         GUIStyle imageStyle = new GUIStyle(); imageStyle.normal.background = texture;
         GUI.Label(imgRect, GUIContent.none, imageStyle);
 
         GUI.BeginGroup(imgRect);
 
         GUI.matrix = oldMatrix;
         var e = Event.current;
         if (e.type == EventType.MouseDown && e.button == 0)
         {
             this.Paint(GUIUtility.ScreenToGUIPoint(e.mousePosition));
         }
         else if (e.type == EventType.ScrollWheel)
         {
             var zoomDelta = 0.1f;
             zoomDelta = e.delta.y < 0 ? zoomDelta : -zoomDelta;
             zoomScale += zoomDelta;
             zoomScale = Mathf.Clamp(zoomScale, 0.25f, 1.25f);
             e.Use();
         }
 
         GUI.EndGroup();
     }
     private void Paint(Vector2 point)
     {
         Debug.Log(point);
         for (int x = Convert.ToInt32(point.x) - size; x < Convert.ToInt32(point.x) + size; x++)
         {
             for (int y = Convert.ToInt32(point.y) - size; y < Convert.ToInt32(point.y) + size; y++)
             {
                 texture.SetPixel(x, y, new Color32(0, 0, 0, 1));
             }
         }
         texture.Apply();
         this.Repaint();
     }
 }