A good tool for dumping media content streamed over rtmp is rtmpdump and a good tool for extracting frames from a video is ffmpeg. Luckily for us those two can be piped together, since rtmpdump by default dumps the streamed media content to standard output and ffmpeg provides the option to read the input from a pipe.
To extract one frame per second from a stream try something like this:
rtmpdump -r <rtmp_video_url> | ffmpeg -i pipe:0 -r 1 frame%05d.jpg