In addition to command line tools such as man sips, ImageMagick or GIMP (batch mode) there's CoreImageTool to edit images from the command line using Apple's CoreImage framework (Mac OS X 10.4 and Mac OS X 10.5).

# download & install CoreImageTool

# create /usr/local/bin
/usr/bin/sudo /bin/mkdir -p /usr/local/bin
/usr/bin/sudo /usr/sbin/chown root:wheel /usr/local /usr/local/bin
/usr/bin/sudo /bin/chmod 0755 /usr/local /usr/local/bin

cd ~/Desktop
/usr/bin/curl -L -O http://www2.entropy.ch/download/CoreImageTool
/usr/bin/sudo /bin/mv -i ~/Desktop/CoreImageTool /usr/local/bin

/usr/bin/sudo /usr/sbin/chown root:wheel /usr/local/bin/CoreImageTool
/usr/bin/sudo /bin/chmod 755 /usr/local/bin/CoreImageTool

ls -l /usr/local/bin/CoreImageTool


/bin/mkdir -p ~/Desktop/CoreImageDir
/bin/cp /Library/Desktop\ Pictures/Nature/Ladybug.jpg ~/Desktop/CoreImageDir

chmod 0777 ~/Desktop/CoreImageDir/Ladybug.jpg
ls -l ~/Desktop/CoreImageDir/Ladybug.jpg

cd ~/Desktop/CoreImageDir


# process Ladybug.jpg

# convert .jpg to .png
CoreImageTool load pic Ladybug.jpg store pic Ladybug.png public.png

CoreImageTool \
   load pic Ladybug.jpg \
   store pic Ladybug.png public.png

open -a Preview Ladybug.jpg
open -a Preview Ladybug.png

CoreImageTool load pic Ladybug.jpg filter pic CILanczosScaleTransform scale=0.5 store pic Ladybug2.jpg public.jpeg
open -a Preview Ladybug2.jpg

CoreImageTool load pic Ladybug.jpg filter pic CILanczosScaleTransform scale=0.1 store pic Ladybug3.jpg public.jpeg
open -a Preview Ladybug3.jpg

CoreImageTool load pic Ladybug.jpg filter pic CICrystallize radius=6.0 store pic Ladybug4.jpg public.jpeg
CoreImageTool load pic Ladybug.jpg filter pic CICrystallize radius=12.0 store pic Ladybug4.jpg public.jpeg
open -a Preview Ladybug4.jpg

CoreImageTool load pic Ladybug.jpg filter pic CICrop rectangle=250,250,800,800 store pic Ladybug5.jpg public.jpeg
open -a Preview Ladybug5.jpg

# sharpen image
CoreImageTool load pic Ladybug.jpg filter pic CISharpenLuminance sharpness=3 store pic Ladybug6.jpg public.jpeg
open -a Preview Ladybug6.jpg

CoreImageTool load pic Ladybug.jpg filter pic CIUnsharpMask radius=1.0:intensity=2.0 store pic Ladybug7.jpg public.jpeg
open -a Preview Ladybug7.jpg

Further information:

- CoreImageTool
- Apple's CoreImage power in the Terminal
- Apple Core Image Filter Reference
- Developing with Core Image

For more information see Terminal window commands, Discover tput and Apple Shell Scripting Primer -> ANSI Escape Sequence Tables.

tput lines
tput cols

echo $LINES
echo $COLUMNS

stty size
stty size | awk '{print $1}'    # lines
stty size | awk '{print $NF}'   # columns

stty size | cut -d" " -f1   # lines
stty size | cut -d" " -f2   # columns 

stty -a | awk '/rows/ {print $4}'      # lines
stty -a | awk '/columns/ {print $6}'   # columns

stty -a | sed -E -n -e 's/^.*[^[:digit:]]([[:digit:]]+)[[:space:]]+rows;.*$/\1/p;q;'
stty -a | sed -E -n -e 's/^.*[^[:digit:]]([[:digit:]]+)[[:space:]]+columns;.*$/\1/p;q;'



# automatically resize the Terminal window if it gets smaller than the default size

# positive integer test (including zero)
function positive_int() { return $(test "$@" -eq "$@" > /dev/null 2>&1 && test "$@" -ge 0 > /dev/null 2>&1); }


# resize the Terminal window
function sizetw() { 
   if [[ $# -eq 2 ]] && $(positive_int "$1") && $(positive_int "$2"); then 
      printf "\e[8;${1};${2};t"
      return 0
   fi
   return 1
}


# the default Terminal window size: 26 lines and 107 columns
sizetw 26 107


# automatically adjust Terminal window size
function defaultwindow() {

   DEFAULTLINES=26
   DEFAULTCOLUMNS=107

   if [[ $(/usr/bin/tput lines) -lt $DEFAULTLINES ]] && [[ $(/usr/bin/tput cols) -lt $DEFAULTCOLUMNS ]]; then
      sizetw $DEFAULTLINES $DEFAULTCOLUMNS
   elif [[ $(/usr/bin/tput lines) -lt $DEFAULTLINES ]]; then
      sizetw $DEFAULTLINES $(/usr/bin/tput cols)
   elif [[ $(/usr/bin/tput cols) -lt $DEFAULTCOLUMNS ]]; then
      sizetw $(/usr/bin/tput lines) $DEFAULTCOLUMNS
   fi

   return 0
}


# SIGWINCH is the window change signal
trap defaultwindow SIGWINCH    


sizetw 26 70
sizetw 10 107
sizetw 4 15

Resizes an image to fit into its container, depending on whether the width or height is longer, yet keeping the aspect ratio. Copied from a comment on sitepoint.com.

ffunction resizeImgOO(el) 
{ 
	function imgRatio()
	{ 
		return (el.height / el.width); 
	} 
	function holderRatio()
	{ 
		return (el.offsetParent.offsetHeight / el.offsetParent.offsetWidth); 
	} 
	function fitToContainer()
	{ 
		if(imgRatio>holderRatio) 
		{ 
			el.height = el.offsetParent.offsetHeight; 
		} 
		else 
		{ 
			el.width = el.offsetParent.offsetWidth;	
		} 
	}
	
	this.imgRatio = imgRatio; 
	this.holderRatio = holderRatio; 
	this.resize = fitToContainer; 
}
var img = new resizeImgOO(document.getElementById('yourImgId'));
img.resize();

Resize an image, keeping proportions to a new width or a new height. oh, and it caches.
You'll need to setup a writable directory $_SERVER[ 'DOCUMENT_ROOT' ].'/images/cache/ to use it.

<?php

if (isset($_GET[ 'image' ]))
{
    $image = realpath($_SERVER[ 'DOCUMENT_ROOT' ].urldecode($_GET[ 'image' ]));
    
    if (!file_exists($image))
    {
        die ( 'no such image!' );
    }
} else {
    die ( 'Missing argument!' );
}

function between($x, $y, $z)
{
    return $x >= $y && $x <= $z;
}

function cachefilename( $o, $w, $h)
{
    return $_SERVER[ 'DOCUMENT_ROOT' ].'/images/cache/'.base64_encode( $o.$w.$h ).'jpeg';
}

$imageinfo = getimagesize($image);
$w = $imageinfo[0];
$h = $imageinfo[1];
// the 64 and 1024 are minimum and maximum, change to please
if (isset ($_GET[ 'width' ]) && between($_GET[ 'width' ], 64, 128))
{
    $resizedwidth = $_GET[ 'width' ];
    $wf = $w / $resizedwidth;
    $resizedheight = $h / $wf;

} else {
    // same here
    if (isset ($_GET[ 'height' ]) && between($_GET[ 'height' ], 64, 1024))
    {
        $resizedheight = $_GET[ 'height' ];
        $hf = $h / $resizedheight;
        $resizedwidth = $w / $hf;

    } else {
        $resizedwidth = 600;
        $wf = $w / $resizedwidth;
        $resizedheight = $h / $wf;
    }
}

if (file_exists( cachefilename($image, $resizedwidth, $resizedheight) ))
{
    header ("Content-type: image/jpeg");
    readfile(cachefilename($image, $resizedwidth, $resizedheight) );
} else {
    $img = imagecreatefromstring(file_get_contents( $image ));
    $thumb = imagecreatetruecolor($resizedwidth, $resizedheight);
    imagecopyresampled($thumb, $img, 0, 0, 0, 0, $resizedwidth, $resizedheight, $w, $h);
    header ("Content-type: image/jpeg");
    imagejpeg($thumb, cachefilename($image, $resizedwidth, $resizedheight), 70);
    readfile(cachefilename($image, $resizedwidth, $resizedheight) );
    imagedestroy($img);
    imagedestroy($thumb);
}

?>