Something

#!/bin/bash

PHOTODIR="/home/clare/Pictures/pics"
SIZE="640x480"

echo "Searching ${PHOTODIR} for pictures to resize..."
echo 'Files with the prefix of "_" will be resized.'
PHOTOLIST=`find $PHOTODIR -iname _\*.jpg -printf "%h%%%f\n"`
echo "Beginning resize loop..."
for PHOTOFIND in $PHOTOLIST
do
  PHOTODIR=`echo ${PHOTOFIND} | cut -f 1 -d "%"`
  PHOTOFILE=`echo ${PHOTOFIND} | cut -f 2 -d "%"`
  OLDPHOTO="${PHOTODIR}/${PHOTOFILE}"
  NEWPHOTO="${PHOTODIR}/(${SIZE})${PHOTOFILE}"
  if [ ! -f "${NEWPHOTO}" ]
    then
      echo "${OLDPHOTO} -> ${NEWPHOTO}"
      convert -size ${SIZE} $OLDPHOTO -resize ${SIZE} ${NEWPHOTO}
  fi
done
echo "Done resizing images.  You may now close this window."