Image list performance ISSUE

Hi, I have tons of images, which belongs to different users. In the software we show to the users only the images that they have, but If they take the image links manually they can also see the other images if they modify the image name in the link. Now, I can override the image request with apache rewrite and send the request to a php file, which analizes the user rights and if the user has rights to see that image. After that I output the image file with php this way: header("Content-Type: image/jpeg"); header("Content-Length: ".filesize($fname)); readfile($fname); ... where fname is the image file. I want to know... is this a big performance issue or not(the image is handled by php and not by apache directly) OR... Is there any other way to handle this situation??? Thanx, Andy.