//The first thing to do is to paste this code on your functions.php file:

function image_shortcode($atts, $content = null) {
    extract( shortcode_atts( array(
    'name' => '',
    'align' => 'right',
    'ext' => 'png',
    'path' => '/wp-content/uploads/',
    'url' => ''
    ), $atts ) );
    $file=ABSPATH."$path$name.$ext";
    if (file_exists($file)) {
        $size=getimagesize($file);
        if ($size!==false) $size=$size[3];
        $output = "<img src='".get_option('siteurl')."$path$name.$ext' alt='$name' $size align='$align' class='align$align' />";
        if ($url) $output = "<a href='$url' title='$name'>".$output.'</a>';
        return $output;
    }
    else {
        trigger_error("'$path$name.$ext' image not found", E_USER_WARNING);
        return '';
    }
}
add_shortcode('image','image_shortcode');

/*
Once done, you can use the image shortcode in your posts. For example, the following line of code: [image name=cat]
*/

http://www.wprecipes.com/wordpress-shortcode-automatically-insert-image-by-file-name

WordPress shortcode:  Automatically insert image by file name

wordpress

PHP