I got stuck with the problem...

I like to create a picture based subtitle (e.g. Vobsub) and mux it together with a HD H.264 video into a mp4 container.

I am able to create the single images and also a .xml or other file with the timing information with a script.

But how do I combine my single images to a format that for example MP4Box can mux it into a MP4 file?