ExternalException "A generic error occurred in GDI+."

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C#

I'm developing a project of image merging.(Image Combine) 1st foreground image is on the(0,0) position of background image. The position is changing by using a numericUpdown Control. The code is below

private void nudXPositions_ValueChanged(object sender, EventArgs e) 
{
    {
        lock (typeof(FrmImageMerge))
        {
            nudXPositions.Maximum = imgBackWidth;

            if (pbMergeImagePreview.Image != null)
            {
                pbMergeImagePreview.Image.Dispose();
            }

            pbMergeImagePreview.InitialImage = null;

            posX = decimal.ToInt16(nudXPositions.Value);
            posY = decimal.ToInt16(nudYPosition.Value);
            MergeImages(tbxBackImage.Text, tbxForeImage.Text);
            lblMergeImagePreview.Text = "";
            pbBackgroundImagePreview.Refresh();
            try
            {
                Thread.Sleep(100);
                Image image1 = Image.FromFile(tempName);
                this.pbMergeImagePreview.Image = image1;
                pbBackgroundImagePreview.Refresh();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

            btnSaveImageAs.Enabled = true;
        }
    }
}

then call MergeImage() method. That method is below

private void MergeImages(string ImageBack,string ImageFore) 
{ 
    pbMergeImagePreview.Image = null; pbMergeImagePreview.Refresh(); 
    //try 
    //{ 
    backExtension = Path.GetExtension(tbxBackImage.Text); 
        System.Drawing.Graphics myGraphic = null; 
        Image imgB; imgB = Image.FromFile(ImageBack); 
        Image imgF; imgF = Image.FromFile(ImageFore); 
        Image m; m = Image.FromFile(ImageBack); 
        myGraphic = System.Drawing.Graphics.FromImage(m); 
        myGraphic.DrawImageUnscaled(imgB,0,0); 
        myGraphic.DrawImageUnscaled(imgF,posX,posY);

        myGraphic.Save();
        getMyDocument = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
        tempName = string.Format("{0}{1}{2}{3}{4}{5}",
                                 getMyDocument,
                                 "\\",
                                 "Merge Image",
                                 "\\",
                                 "Imageback", 
                                 backExtension);

        if (File.Exists(tempName))
        {
            File.Delete(tempName);
        }
        m.Save(tempName);       

        imgB.Dispose();
        imgF.Dispose();
        m.Dispose();
        myGraphic.Dispose();

    //}
    //catch (Exception ex)
    //{
    //    MessageBox.Show(ex.Message);
    //}
}

When program is execute and when change the position ImageFore is change according to the position. It's save to a temporary image. Then If we change the position again temporary image is recreated and it replace to the old image. But when I do this procedure for a some time there will be an exception;

External Exception has Occurred.

Also this is a

A generic error occurred in GDI+.

I tried a lot and search browser. It I couldn't solve my problem. Thank You