A problem of merging images saving

private void MergeImages(string ImageBack,string ImageFore)
		{
                       try
             {
                 string outputFile = Path.GetFileName(tbxImageName.Text.Trim());
                 string fullFile = Path.GetFullPath(tbxImageName.Text);
                 string backFile = Path.GetFileName(tbxBackImage.Text.Trim());
                 string fullBack = Path.GetFullPath(tbxBackImage.Text);
                  System.Drawing.Graphics myGraphic = null;
                 Image imgB;
		  imgB = Image.FromFile(ImageBack);
                 FileInfo fileInfoBack = new FileInfo(ImageBack);
                 FileInfo fileInfoForward=new FileInfo(ImageFore);
                 string aa= Path.GetFullPath(tbxImageName.Text);
                 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();
               int opnselctn = openFileDialog2.FilterIndex;
			    switch(opnselctn)
                {
                   case 1:
                       m.Save(ImageBack.Replace(fullBack, fullFile), System.Drawing.Imaging.ImageFormat.Jpeg);
                     break;
                  case 2:
                      m.Save(ImageBack.Replace(fullBack, fullFile), System.Drawing.Imaging.ImageFormat.Bmp);
                     break;
                  case 3:
                      m.Save(ImageBack.Replace(fullBack, fullFile), System.Drawing.Imaging.ImageFormat.Gif);
                    break;
                  case 4:
                    m.Save(ImageBack.Replace(fullBack, fullFile), System.Drawing.Imaging.ImageFormat.Png);
                    break;
                   }
                   imgB.Dispose();
                   imgF.Dispose();
                 m.Dispose();
           }
			catch (Exception ex)
			{
				MessageBox.Show(ex.Message);
			}