If I'm on a kiwi SDR, I run MMSSTV with the soundcard set to internal sound (in my case, on my Windows 10 laptop that setting is "microphone array" but used to be called something else before I recently updated Windows 10 to the latest version). Any SSTV decoding app should have a similar setting. I don't bother with any kiwi SDR settings other than the volume. I haven't any idea what the SSTV extension is supposed to do. I hope this helps.