So basically this[0] with a crossfade[1] between frames? Which would be (by stitching examples together)

  ffmpeg -framerate 1/5 -pattern_type glob -i '\*.jpg' -pix_fmt yuv420p -filter_complex xfade=transition=fade:duration=2:offset=2 out.mp4

(1 jpg image per 5 seconds with a 2 seconds crossfade in between)

[0] https://trac.ffmpeg.org/wiki/Slideshow

[1] https://ffmpeg.org/ffmpeg-filters.html#xfade (only available in >4.3)