Hi,

I'd like to know if there exist some plugin to kdenlive for rendering into image sequence. I browsed setts and internet and it seems to me this is the weak point of the program.

You can render to PNG and TIFF sequence as described here:
http://kdenlive.org/forum/export-video-image-sequence

You can do it easily with ffmpeg on the command line. I couldn't find a built-in option to do it in Kdenlive, but since Kdenlive uses ffmpeg for rendering, you should be able to create a custom rendering profile from the ffmpeg commands.

Quoted from the FFMpeg docs:

For extracting images from a video:
ffmpeg -i foo.avi -r 1 -s WxH -f image2 foo-%03d.jpeg
This will extract one video frame per second from the video and will output them in files named ‘foo-001.jpeg’, ‘foo-002.jpeg’, etc. Images will be rescaled to fit the new WxH values.

You would typically want to replace the -r option with whatever frame rate your video has. Also, maybe using %04d or something higher than %03d would make sense if you need more than 1,000 images (=only 40 seconds of video at 25fps).