Luke,
How are you creating the image? The following bit of code (taken from the manual) creates a png image with the text specified.:
header ("Content-type: image/png");
$im = @ImageCreate (300, 50)
or die ("Cannot Initialize new GD image stream");
$background_color = ImageColorAllocate ($im, 255, 255, 255);
$text_color = ImageColorAllocate ($im, 233, 14, 91);
ImageString ($im, 1, 5, 5, "A Simple Text String", $text_color);
ImagePng ($im);
Are you doing something different? - Could you please paste the code you're using?
-mustafa