Convert images to many formats. Quality can be set. The converted images will be placed in a separate folder and the originals will be intact. Zenity must be installed.

convert-image.sh

#! /bin/sh                                                      

# Increment and count the number of files passed to the script
# for later use in the progress dialog box
n=1

# Ask for the destination graphic image format to change to
# Next line is entered all on one line
EXT=`zenity --title="Convert Image" --entry --text="Please 
enter the image format to convert to (e.g. PNG, JPG, TIFF, WEBP etc.)"`

  if [ $? == 1 ]; then

     exit
  fi

# Make sure the file extension is in lowercase text
declare -l EXT
EXT=$EXT

# Ask for the quality level of the copied images
# Next line is entered all on one line
QUAL=`zenity --title="Please enter quality" --entry --text="100 
= Full Quality, 75 = 75% Quality, 50 = 50% Quality"`

  if [ $? == 1 ]; then

     exit
  fi

# If the destination image format is PNG, set background to none
# (transparent) and set the quality levels, which differ from JPG
if [ $EXT == "png" ];
  then
     BACKGROUND="-background none"
     QUALITY=$(($QUAL / 10))
  else
     BACKGROUND="-background white"
     QUALITY="$QUAL"
fi

# Generate the new directory name, and set it to uppercase text
NEW_DIR="./"$EXT"-"$(date +"%m%d%y-%H%M%S")
declare -u NEW_DIR
NEW_DIR=$NEW_DIR
mkdir $NEW_DIR

# Pause for one second, just to be sure that everything that
# needs to be done has been done
sleep 1

# Cycle through the selected files, performing the conversion
# one file at a time
for file in $@; do
    if [ ! -e $file ]; then
       continue
    fi

     # Get just the filename, without the original file extension
    name=$( echo $file | cut -f1 -d.)

     # Convert the file and write it out to the destination file
     # Next line is entered all on one line
    convert -quality $QUALITY $BACKGROUND $file $NEW_DIR/${name}.$EXT

     # Set up the information to display in the progress dialog box
    echo $(($n * 100 / $#))
    echo "# Processing file: $file"
    let "n = n+1"

# Next line is entered all on one line
done | (zenity  --progress --title "Converting to "$EXT"..." --percentage=0 --auto-close --auto-kill)

exit 0

HI, using this is creating an empty folder. and no progress bar is showing either.