I have found a solution to the problem now. If anyone wants to know, I wrote a small program that uses rtmpdump and ffmpeg to capture an image.
static void Main(string[] args) { const string rtmpDump = "rtmpdump.exe"; const string rtmpDumpArguments = "-v -r rtmp://{stream} -o 1.flv -B 1"; sRunExternalExe(rtmpDump, rtmpDumpArguments); const string ffmpeg = "ffmpeg.exe"; const string ffmpegArguments = "-i 1.flv -ss 00:00:01 -an -r 1 -vframes 1 -s 400x300 -y 1.jpg"; RunExternalExe(ffmpeg, ffmpegArguments); var theFile = new FileInfo("1.flv"); if (theFile.Exists) { File.Delete("1.flv"); } } public static string RunExternalExe(string filename, string arguments = null) { var process = new Process(); process.StartInfo.FileName = filename; if (!string.IsNullOrEmpty(arguments)) { process.StartInfo.Arguments = arguments; } process.StartInfo.CreateNoWindow = true; process.StartInfo.WindowStyle = ProcessWindowStyle.Hidden; process.StartInfo.UseShellExecute = false; process.StartInfo.RedirectStandardError = true; process.StartInfo.RedirectStandardOutput = true; var stdOutput = new StringBuilder(); process.OutputDataReceived += (sender, args) => stdOutput.Append(args.Data); string stdError = null; try { process.Start(); process.BeginOutputReadLine(); stdError = process.StandardError.ReadToEnd(); process.WaitForExit(); } catch (Exception e) { throw new Exception("OS error while executing " + Format(filename, arguments) + ": " + e.Message, e); } if (process.ExitCode == 0 || process.ExitCode == 2) { return stdOutput.ToString(); } else { var message = new StringBuilder(); if (!string.IsNullOrEmpty(stdError)) { message.AppendLine(stdError); } if (stdOutput.Length != 0) { message.AppendLine("Std output:"); message.AppendLine(stdOutput.ToString()); } throw new Exception(Format(filename, arguments) + " finished with exit code = " + process.ExitCode + ": " + message); } } private static string Format(string filename, string arguments) { return "'" + filename + ((string.IsNullOrEmpty(arguments)) ? string.Empty : " " + arguments) + "'"; }