How to fiqure out my problem with picture box in thread?

public int[] arr1;
private void button1_Click(object sender, EventArgs e)
{
    if (InitializingData())
    {
        label1.Visible = true;
        Thread t1 = new Thread(delegate() { MergeSort(arr1); });
        t1.Start();
    }
}

private bool InitializingData()
{
    if (!File.Exists(Application.StartupPath + @"\input.txt"))
    {
        MessageBox.Show("The input file was not found!");
        return false;
    }
    try
    {
        #region Reading input file as a string array, and reserving space for input data
        StreamReader objFileReader = new StreamReader(Application.StartupPath + @"\input.txt");
        string[] str = objFileReader.ReadLine().Split(',');
        objFileReader.Dispose();
        arr1 = new int[str.Length];
        arr2 = new int[str.Length];
        arr3 = new int[str.Length];
        MessageBox.Show("The usage space was reserved successfully!!!");
        #endregion

        #region Putting input elements from the string array to integer array
        int cnt = 0;
        foreach (string obj in str)
        {
            arr1[cnt] = int.Parse(obj);
            arr2[cnt] = int.Parse(obj);
            arr3[cnt++] = int.Parse(obj);
        }
        MessageBox.Show("The input data initialized successfully!!!");
        #endregion
    }
    catch
    {
        MessageBox.Show("Wrong input!!!");
        return false;
    }
    return true;
}

#region Merge Sort...
private void MergeSort(int[] l_arr)
{
    int h = l_arr.Length / 2;
    int m = l_arr.Length - h;
    int[] left = new int[h];
    int[] right = new int[m];
    if (l_arr.Length > 1)
    {
        for (int i = 0; i < h; i++)
            left[i] = l_arr[i];
        for (int i = h, j = 0; i < l_arr.Length; i++, j++)
            right[j] = l_arr[i];

        MergeSort(left);
        MergeSort(right);
        Merge(left, right, l_arr);
    }
}

private void Merge(int[] left, int[] right, int[] l_arr)
{
    int arrIndex, leftIndex, rightIndex;
    arrIndex = leftIndex = rightIndex = 0;
    while (leftIndex < left.Length && rightIndex < right.Length)
    {
        if (left[leftIndex] < right[rightIndex])
            l_arr[arrIndex++] = left[leftIndex++];
        else
            l_arr[arrIndex++] = right[rightIndex++];
    }
    while (leftIndex < left.Length)
        l_arr[arrIndex++] = left[leftIndex++];
    while (rightIndex < right.Length)
        l_arr[arrIndex++] = right[rightIndex++];
    ShowProgressBar(pictureBox1, arr1.Length);
}
#endregion

#region Show progress bar
int cnt = 1;
private void ShowProgressBar(object sender, int max)
{
    PictureBox objProgressBar = (PictureBox)sender;
    Bitmap objBM = new Bitmap(objProgressBar.Width, objProgressBar.Height);
    Graphics objGraphic = Graphics.FromImage(objBM);
    objGraphic.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
    objGraphic.FillRectangle(Brushes.Violet, new Rectangle(0, 0, ((cnt++) * objProgressBar.Width) / max, objProgressBar.Height));
    try { objProgressBar.Image = objBM; }
    catch { }
    Application.DoEvents();
}
#endregion

int cnt = 1;
        private void ShowProgressBar(object sender, int max)
        {
            
            if (InvokeRequired)
            {
                Invoke(new MethodInvoker(delegate { ShowProgressBar(sender, max); }));
            }
            else
            {
                PictureBox objProgressBar = (PictureBox)sender;
                Bitmap objBM = new Bitmap(objProgressBar.Width, objProgressBar.Height);
                Graphics objGraphic = Graphics.FromImage(objBM);
                objGraphic.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                objGraphic.FillRectangle(Brushes.Violet, new Rectangle(0, 0, ((cnt++) * objProgressBar.Width) / max, objProgressBar.Height));
                objProgressBar.Image = objBM;
                this.Text = (((cnt) * 100) / max).ToString() + '%';
            }
        }

#region Quick Sort
        int pivotpoint;
        void QuickSort(int low, int high, int[] l_arr)
        {
            pivotpoint=low;
            if (high > low)
            {
                partition(low, high, l_arr);
                QuickSort(low, pivotpoint - 1, l_arr);
                QuickSort(pivotpoint + 1, high, l_arr);
            }
        }
        void partition(int low, int high, int[] l_arr)
        {
            int i, j;//, pivotpoint;
            int tmp;
            int pivotitem = l_arr[low];
            j = low;
            for (i = low + 1; i <= high; i++)
            {
                if (l_arr[i] < pivotitem)
                {
                    j++;
                    tmp = l_arr[i];
                    l_arr[i] = l_arr[j];
                    l_arr[j] = tmp;
                }
            }
            pivotpoint = j;
            tmp = l_arr[low];
            l_arr[low] = l_arr[pivotpoint];
            l_arr[pivotpoint] = tmp;
            if (InvokeRequired)
            {
                Invoke(new MethodInvoker(delegate { partition(low, high, l_arr); }));
            }
            else
            {
                ShowProgressBar(pictureBox2, arr2.Length);
            }
        }
        #endregion