How to protect against direct access to images?

http://us3.php.net/image

You link the img element to a php file. This file checks if the user has the right permission, if so it can send an img response back.

<img src="url/LoadImg.php?id=1337" alt="" />

Still someone with the permission can download the image and provide it to other people somewhere else (webspace/mail/whatever). To make it a bit harder to steal it you can disable right clicking on the image, but still a user who knows a little bit about http should not have any problems to steal it. You can place a signature over the image (for example the logo/name of your website) so people can see that you where the source. This can be done with php aswell.

If you want to be funy you can setup an other image that is sent if the link comes from an other page :P

Add a simple .htaccess file in your site folder with the follwoing lines

RewriteEngine on
RewriteCond %{HTTP_REFERER} !^http://www\.your-domain\.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www\.your-domain\.com$ [NC]
RewriteRule .*\.(wav|swf|jpg|jpeg|gif|png|bmp|js|css)$ - [F,NC,L]

Note I added also js and css file even if I think it's bizzare to find someone who attempts to scrape them.

You can dynamically protect a folder using htaccess and the users ip.

Add a .htaccess file to your images folder with the following lines:

order deny,allow
deny from all

Then use PHP to insert the users ip into the htaccess file when they log in like this:

<?
$ip = $_SERVER['REMOTE_ADDR'];
if (!filter_var($ip, FILTER_VALIDATE_IP)) exit();
$file = $_SERVER["DOCUMENT_ROOT"].'/YOUR_IMAGE_FOLDER/.htaccess';
$current = file_get_contents($file);
$current .= "allow from ".$_SERVER['REMOTE_ADDR']." #".$_SESSION['id']."\n";
file_put_contents($file, $current);
?>

The folder will be blocked from any ip that is not logged in.

Notice that I checked to see if the ip is valid. It is important that you give the user no way to inject their own code into your htaccess file.

Also notice that I put the users id in a comment to the right of the ip in the htaccess file. When the user logs out you can search the htaccess file and remove the ip of the user.

You can update this on every request to prevent users who are using dynamic ips from getting kicked off.

I use this method with my entire members areas, it is an excellent added layer of security. Just make sure that you put your log in scripts outside of the members folder.

You could use a PHP script to retrieve the images using something like:

<img src="mysite.com/getimage.php?id=001" />

and have the PHP script return the image data only after confirming that the domain of the HTTP_REFERER is your's.

If you have an account-oriented site, I suggest using PHP sessions as you stated and have the PHP script verify the session before returning the image data.

This might be useful: Allow/deny image hotlinking with .htaccess

Edit: One thing to note about this method is that some Browser/AV/Firewall software removes Referer data when you browse, which would cause potentially legitimate users to be treated as hotlinkers.

If your site already uses some kind of authentication or session system, then it would be better to use the method given in @Mark Baijens' answer.

Update: NGiNX rewrite rule to prevent hotlinking:

location ~* (\.jpg|\.png|\.css)$ {
    valid_referers blocked mydomain.com www.mydomain.com;
    if ($invalid_referer) {
        return 444;
    }
}