GD wouldn't be useful since it only creates images and doesn't read them.

Not so. GD happily loads existing images in the usual formats:

#! perl -slw
use strict;
use GD;

my $img = GD::Image->new( $ARGV[ 0 ] );
my( $w, $h ) = $img->getBounds;
print "The image '$ARGV[ 0 ]' is $w x $h pixels.";

printf "The color of the pixel in the middle is: [0x%02x:0x%02x:0x%02x
+].\n",
    $img->rgb( $img->getPixel( $w/2, $h/2 ) );

__DATA__
c:\test>junk5 694790clustered.png
The image '694790clustered.png' is 800 x 800 pixels.
The color of the pixel in the middle is: [0xfc:0xfe:0xfc].

c:\test>junk5 img\worldx2-2.jpg
The image 'img\worldx2-2.jpg' is 5400 x 2700 pixels.
The color of the pixel in the middle is: [0x04:0x0c:0x11].

c:\test>junk5 img\gradient.gif
The image 'img\gradient.gif' is 108 x 108 pixels.
The color of the pixel in the middle is: [0xff:0xff:0xff].
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