My app jams

Question

hi all,

 I have just wrote my first app in WPF (actually it is my first windows app). It is an image resizer which converts pictures correctly but it jams when I load large number of images (The window freezes and I cant move it). 

I have a list of strings where each item represents location of the image. Then I run the following loop:

 foreach (string file in fileCollection)


            {
		index++;
                FileInfo fInfo = new FileInfo(file);

                Image tempImg2 = Image.FromFile(fInfo.FullName);

                string imgName = fInfo.Name.Substring(0, fInfo.Name.Length - 4);

                if (isOriginalSize == true)

                {

                    finalImage = tempImg2;  
 
                }

              else

                {

                    finalImage = tempImg2.GetThumbnailImage(imageWidth, imageHeight, null, IntPtr.Zero);

                }

                        Bitmap bmp1 = new Bitmap(finalImage);

                        tempImg2.Dispose();

                        finalImage.Dispose();

                        ImageCodecInfo jpgEncoder = GetEncoder(ImageFormat.Jpeg);

                        System.Drawing.Imaging.Encoder myEncoder = System.Drawing.Imaging.Encoder.Quality;

                        EncoderParameters myEncoderParameters = new EncoderParameters(1);

                        EncoderParameter myEncoderParameter = new EncoderParameter(myEncoder, imgQualityValue);

                        myEncoderParameters.Param[0] = myEncoderParameter;



                        bmp1.Save(@ofd_out.SelectedPath + "\\" + imgName + outputFormat, jpgEncoder, myEncoderParameters);

                        bmp1.Dispose();


                }

What can I do to improve performance of my program? Should I use threading or something else? 

Thank you,

Marcin