Convert

Imagemagick Command – Convert

## get image info
identify $filename

# resize
## ! can force-cast to the required size  
convert -resize 160x80! $infile $outfile 

# quality
## compress with quality, generally 60 -80  
convert -quality [0-100] $infile $outfile

e.g, resize jpg in current file

#!/bin/bash
for file in `ls *.jpg`; do
	convert $file -resize 256x160! -quality 30 $file
done