Here is a simple php dynamic image script I just whipped up which can pull a string from a text file. The problem with it lies in the fact that image dimensions must be defined in the script, so of course varied string lengths and/or text sizes will pose problems in image dimensions which will then still need to be edited when ever you update the string.. so not really going to work for ya..but thought i'd share it any way.
PHP Code:
<?php
header("Content-type: image/jpeg");
//define txt file path
$file = 'string.txt';
//define font path
$font = 'arialbd.ttf';
//get string from txt file
if($mystring=file_get_contents($file))
//create image, parameters (width,height)
$img = imagecreate(325,25);
//define colors and fill
$bgcolor = imagecolorallocate($img,255,255,255);
$textcolor = imagecolorallocate($img,100,100,100);
imagefill($img,0,0,$bgcolor);
//add string to image
//parameters are (image, font-size, angle, x-coordinate, y-coordinate, color, font, string)
imagettftext($img, 20, 0, 10, 20, $textcolor, $font, $mystring);
imagejpeg($img);
imagedestroy($img);
?>
But if I could see your current script, it'd probably be possible to edit in changes to allow it to pull from a text file. So if you'd like for me to try that, let me know 
Cheers!
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