How to release memory of image controls which are not in "field of view" of listview

Question

I have a listview with Image control, bound with Uri value (image path on disk). I want to convert only those Uri in "Field of view" and dispose (or convert back to Uri) which goes out of "Field of View".

What I have tried:

I have used IValueConverter to convert Uri to Image but ConvertBack is not firing.

<Image Source="{Binding PageSource,Mode=TwoWay, UpdateSourceTrigger=Explicit, Converter={StaticResource ImageConverter}}" HorizontalAlignment="Center" VerticalAlignment="Top"/>

Source code:

    private void LoadPdfFile(string fileName)
    {
        string folderPath = Environment.CurrentDirectory + "//ImageCache";
        Directory.CreateDirectory(folderPath);

        //concurrent queue- thread safe
        ConcurrentQueue<PdfPage> cQueue = new ConcurrentQueue<PdfPage>();

        //loading pdf file
        using (FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read))
        {
            //syncfusion pdf library to load pdf file
            using (PdfLoadedDocument ldoc = new PdfLoadedDocument(fs))
            {
                //parallel for- to simultaneous loop
                Parallel.For(0, ldoc.Pages.Count,
                         i =>
                         {
                             //convert pdf page into bitmap
                             Bitmap img = ldoc.ExportAsImage(i);

                             //saving image onto disk
                             //I don't want to load it into memory directly to save memory(but it is not working)
                             string imageName = folderPath + "//" + Guid.NewGuid().ToString();
                             img.Save(imageName);

                             //filling object value
                             PdfPage newFile = new PdfPage();
                             //Uri PageSource
                             newFile.PageSource = new Uri(imageName, UriKind.Absolute);
                             //int PageNumber
                             newFile.PageNumber = i + 1;
                             cQueue.Enqueue(newFile);
                             //disposing bitmap value
                             img.Dispose();
                             img = null;
                             GC.Collect();
                             GC.WaitForPendingFinalizers();
                         });
            }
        }

        List<PdfPage> Pages = cQueue.OrderBy(x => x.PageNumber).ToList();
        //assigning it to the collectionviewsource
        itemCollectionViewSource.Source = Pages;
    }
    public class ImageConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value == null)
            {
                return null;
            }
            else
            {
                var uri = (Uri)value;
                BitmapImage bmp = new BitmapImage();
                bmp.BeginInit();
                bmp.UriSource = uri;
                bmp.UriCachePolicy = new System.Net.Cache.RequestCachePolicy(System.Net.Cache.RequestCacheLevel.NoCacheNoStore);
                bmp.CacheOption = BitmapCacheOption.None;
                bmp.EndInit();
                return bmp;
            }
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return null;
        }
    }

Update: I have changed the code to

Parallel.For(0, ldoc.Pages.Count,
                             i =>
                             {
                                 using (Bitmap img = ldoc.ExportAsImage(i))
                                 {
                                     using (Stream stream = new MemoryStream())
                                     {
                                         PdfPage newFile = new PdfPage();

                                         img.Save(stream, System.Drawing.Imaging.ImageFormat.Bmp);

                                         BitmapImage bmp = new BitmapImage();
                                         bmp.BeginInit();
                                         bmp.CacheOption = BitmapCacheOption.OnLoad;
                                         bmp.StreamSource = stream;
                                         bmp.EndInit();
                                         bmp.Freeze();
                                         //BitmapImage PageSource
                                         newFile.PageSource = bmp;

                                         newFile.PageNumber = i + 1;
                                         cQueue.Enqueue(newFile);
                                     }
                                 }
                                 GC.Collect();
                                 GC.WaitForPendingFinalizers();
                             });    

but this throws A generic error occurred in GDI+ error for large pages pdf file.