ConvertToWebP.cs

// At first you should download "cwebp.exe".
// The latest source tree is available at:
// https://chromium.googlesource.com/webm/libwebp
byte[] ConvertJpgToWebP(byte[] jpgBytes, string fileName, string extension)
{
    string jpgFilePath = $"images//{fileName}.{extension}";
    string webpFilePath = $"images//{fileName}.webp";

    // Write the JPG byte[] to a file
    File.WriteAllBytes(jpgFilePath, jpgBytes);

    // Invoke the cwebp command-line tool
    ProcessStartInfo startInfo = new ProcessStartInfo
    {
        FileName = "cwebp",
        Arguments = $"\"{jpgFilePath}\" -o \"{webpFilePath}\"",
        RedirectStandardOutput = true,
        UseShellExecute = false,
        CreateNoWindow = true
    };

    using (Process? process = Process.Start(startInfo))
    {
        if (process == null)
            return [];
        // Wait for process.
        process.WaitForExit();
    }

    // Read the WebP file into a byte[]
    byte[] webpBytes = File.ReadAllBytes(webpFilePath);

    // Delete the temporary files
    File.Delete(jpgFilePath);
    File.Delete(webpFilePath);

    return webpBytes;
}