You can use
convert from ImageMagick together with bash script to resize images inside a directory.
mkdir resize; \
IFS=$(echo -en "\n\b"); \
for i in *; do \
echo $i; convert $i -resize 600^ resize/$i; \
The script will make a new directory named "resize" in current directory, resize all images in current directory to smaller ones of width 600px, and store the resized images into "reisze".
IFS=$(echo -en "\n\b") is a bash trick to handle situations where images file names have spaces.