According to Copying an operating system image to an SD card using Mac OS steps are

Info

diskutil list
(snip)
/dev/disk4 (external, physical):
 #: TYPE NAME SIZE IDENTIFIER
 0: FDisk_partition_scheme *32.0 GB disk4
 1: DOS_FAT_32 NO NAME 32.0 GB disk4s1
(snip)

Unmount

Take disk identifier and unmount disk

#/usr/bin/env bash
for F in `ls *.jpg`
do
  echo "$F"
  magick convert $F -resize 25% out/"$(basename "$F" .jpg).png"
done

https://imagemagick.org/script/convert.php