How to convert all PNGs in a folder to JPG in order to make a video?

Disclaimer: Here is a solution to the questions that you asked that works. I've tested it. However, it's not ideal and I don't recommend it, because this code is dependent on how fast your system is able to perform asynchronous context switching.

1. This code works on JPG, PNG, and a few other formats. I'm not sure why you needed to convert all your JPG files to PNG.
2. The reason why your code only showed the last image in the folder is because you were running all your code synchronously on the UI thread, so it wasn't able to update the UI until your entire "for" loop was done. You need to break up the UI thread by using the async/await pattern. Look it up if you're not familiar with this way of coding.

The code:

using System;
using System.IO;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Media.Imaging;

namespace _69201085
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        public void UpdateImage(string imageName)
        {
            ImportImage.Source = new BitmapImage(new Uri(imageName));
        }

        private async Task ShowFilesInFolder(string workingDirectory)
        {
            var textFile = Path.Combine(workingDirectory, "files.txt");
            if (File.Exists(textFile))
            {
                // Parse the file, then iterate through all of the listed images at 25 FPS
                var lines = await File.ReadAllLinesAsync(textFile);
                foreach (var line in lines)
                {
                    var imageName = Path.Combine(workingDirectory, line.Trim());
                    // Check that the file exists before you try to change the image
                    if (File.Exists(imageName))
                    {
                        // Make sure that the file is one of the approved image formats
                        if (
                            Path.GetExtension(imageName).ToLower() == ".jpg" ||
                            Path.GetExtension(imageName).ToLower() == ".png" ||
                            Path.GetExtension(imageName).ToLower() == ".jpeg" ||
                            Path.GetExtension(imageName).ToLower() == ".jfif" ||
                            Path.GetExtension(imageName).ToLower() == ".gif" ||
                            Path.GetExtension(imageName).ToLower() == ".bmp"
                            )
                        {
                            // Update the image
                            UpdateImage(imageName);

                            // If you want 25 FPS, then wait 40 ms
                            await Task.Delay(40);
                        }
                    }
                }
            }
        }

        private async void Window_Drop(object sender, DragEventArgs e)
        {
            if (e.Data.GetDataPresent(DataFormats.FileDrop))
            {
                string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);

                string folder = Path.GetFullPath(files[0]);
                if (!Directory.Exists(folder))
                {
                    folder = Path.GetDirectoryName(files[0]);
                }

                // Verify that the directory/folder exists
                if (Directory.Exists(folder))
                {
                    await ShowFilesInFolder(folder);
                }
            }
        }
    }
}

And the xaml:

<Window x:Class="_69201085.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800" Drop="Window_Drop" AllowDrop="True">
    <Grid>
        <StackPanel x:Name="FileDropStackPanel">
            <Image x:Name="ImportImage" Stretch="Fill" />
        </StackPanel>
    </Grid>
</Window>

Lastly, the code assumes your "files.txt" is listed without the directory name, only the file names. Like this:

file1.jpg
file2.PNG
file3.bmp
file4.jpg
...

You may use ffmpeg and the techniques described in this SuperUser post to turn your file frames into a movie

ffmpeg -framerate 25 -i frame_%08d.jpg out.mp4

If you're on Windows you might have to double up the % sign