Don't worried man, i see most of the website are doing this. =D... I agree to SketchMedia, if you have a high traffic on your website, save the generate image to the hardisk instead.

If you do not want to save to your hardisk, Query the database for the email in the image.php file, that means that this image.php is only usable by you and useless for other website. That is what i always do.