PHP / Apache中的图像显示权限

I have a PHP site with a login system, and am trying to make a feature where only specific usernames can view particular images. I think what I'm trying to do is more involved than merely changing the .htaccess file, because a) this won't help discern between users that are/aren't allowed to view the image, and b) if someone enters the exact URL of the image (I have a PHP site with a login system, and am t