Images from image: http://superuser.com/questions/939910/

#!/bin/bash
# resizer.sh - resize target image between two resolutions
# accepts file as either first argument or by setting FILEPATH variable

# SETTINGS

SMALLEST_WIDTH=73       # px
LARGEST_WIDTH=1000      # px
FILEPATH=               # set if you don't want to pass in image as argument
NUM_OF_RESOLUTIONS=100  # number of images generated; will generate between
                        # $SMALLEST_WIDTH and $LARGEST_WIDTH
RATIO=                  # set if you want to specify width/height
                        # (eg 1/1, 4/3, 16/9), blank is preserve current ratio


# NOTE: resizing to other aspect ratios may be slow/distorty:
# as per http://www.imagemagick.org/Usage/resize/#noaspect
# Seamless resizing (default) may be preferred, see:
# http://www.imagemagick.org/Usage/resize/#liquid-rescale
# but note it is slower, particularly as images get larger

LIQUID=0

# SCRIPT BELOW

die() { printf "$@\n" 1>&2 ; exit 1; }

if [ -z "$FILEPATH" ]; then
        if [ -z "$1" ]; then die "Need to supply file to work on either as argument or by setting FILEPATH!";
        else FILE="$1";
        fi
else
        FILE="$FILEPATH"
fi

# check file exists and is regular file

if [ ! -e "$FILE" ]; then die "$FILE does not exist!"; fi
if [ ! -f "$FILE" ]; then die "$FILE is not a regular file!"; fi

i=0
step=$(echo "($LARGEST_WIDTH - $SMALLEST_WIDTH) / ($NUM_OF_RESOLUTIONS - 1)" | bc -l)
#printf "Resolution step is: %s\n-------------" "$step"
while [ $i -lt $NUM_OF_RESOLUTIONS ]; do
        # handle ratio
        WIDTH=$(echo "$SMALLEST_WIDTH+($step*$i)" | bc -l)
        if [ -z "$RATIO" ]; then
                #printf "convert %s -resize %s %s\n" "$FILE" "$WIDTH" "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}"
                convert "$FILE" -resize "$WIDTH" "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}"
        else
                HEIGHT=$(echo "$WIDTH * $RATIO" | bc -l)
                if [ "$LIQUID" -eq 0 ]; then
                        # Uncomment convert line for distorted ("squashed") resizing
                        #printf "convert %s -resize %sx%s\! %s\n" "$FILE" "$WIDTH" "$HEIGHT" "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}"
                        convert "$FILE" -resize "$WIDTH"x"$HEIGHT"\! "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}"
                else
                        # Liquid resizing: http://www.imagemagick.org/Usage/resize/#liquid-rescale
                        # fast aspect ration resize first, then liquid
                        #printf "convert %s -resize %s %s\n" "$FILE" "$WIDTH" "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}"
                        convert "$FILE" -resize "$WIDTH" "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}"
                        #printf "%s details are now:\n %s\n" "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}" "$(identify "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}")"
                        #printf "convert %s -liquid-rescale %sx%s\! %s\n" "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}" "$WIDTH" "$HEIGHT" "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}"
                        convert "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}" -liquid-rescale "$WIDTH"x"$HEIGHT"\! "${FILE%.*}-${WIDTH%.*}px.${FILE##*.}"
                fi
        fi
        (( i++ ))
done