I am having a little trouble keeping problems categorized and did not yet look at the previous report of the PNG problem (not MKV though) If you could open an issue in MantisBT for just the PNGs, it will not as easily get lost there.
I'm sorry, it's my fault for the chaos of my messages. Like Andrew said, PNG aren't a real problem. So I'm not going to open an issue. When I had problems with playback I noticed that transitions between two PNGs slowed down even more than usual. Associating this with the terminal's message: "FFMPEG::open_decoder: some stream have bad times: /home/paz/video_editing/prova/1080/background.png" I thought the slowdowns were similar to those with mkv. But Andrew clarified the ideas to me and it's not worth wasting time on PNG. I will instead open an issue on mkv, because I think it is important to be able to use them without problems. Their codecs are now very widespread and have great potential.