How do I force a reload of an image that I stream via RTSP?

[Lupin III.]

I have set up a stream of an image on my harddisk with this command line:
vlc -I dummy --image-duration=-1 --image-fps=1/1 --sout "#transcode{vcodec=mp2v,fps=1,vb=1}:rtp{mux=ts,sdp=rtsp://:8554/my_stream}" C:\Users\User\Desktop\my_image.png

I can connect to this stream with another instance of VLC as expected. The bitrate value "vb" seems to be ignored by the encoder though, as there's no difference in the value set, except the image quality being horrible, when it's left out completely.
I now want the image to be updated once a second (or even better only when the image on the disk changes). Setting image duration to 1 and using "--repeat" is no use because this will close the stream every second and the client will lose connection.

How can I make VLC reload the image?

[Jean-Baptiste Kempf]

vb=1 is too low => bad quality

To keep connection, use --sout-keep.