Unfortunately, --all isn't an option. The following options are available in swayimg:

Mandatory arguments to long options are mandatory for short options too.
  -r, --recursive      read directories recursively
  -o, --order=ORDER    set sort order for image list: none/[alpha]/random
  -s, --scale=SCALE    set initial image scale: [optimal]/fit/width/height/fill/real
  -l, --slideshow      activate slideshow mode on startup
  -f, --fullscreen     show image in full screen mode
  -p, --position=POS   set window position [parent]/X,Y
  -g, --size=SIZE      set window size: [parent]/image/W,H
  -a, --class=NAME     set window class/app_id
  -c, --config=S.K=V   set configuration parameter: section.key=value
  -v, --version        print version info and exit
  -h, --help           print this help and exit