I'm having a project of the digital photographer. He got lots of copyright images and wish to show or preview these questions website with watermark regarding this. Younger crowd really wants to enable download of excellent quality original picture without watermark of this picture towards the people who taken care of that images.

I'm planning as following for stopping unsanctioned download:
1.) placing all of the images with no watermark inside a secret folder and stopping use of its content using .htaccess
2.) to exhibit image with watermark refer to it as using preview.php file [browse the file from secret file and add watermark and show it to browser]
3.) make it possible for download without watermark refer to it as using download.php and inside download.php checking the qualifications and when everything goes right with drenched user then browse the original file from secret location and output this content to browser.

Are there more safety measures which i should take?

What you're explaining here sounds fine. I 'd most likely place the photos entirely outdoors the webroot, but that should not create a practical difference versus the .htaccess.

Rather than adding b .htaccess file, just place the folder outdoors of the websites root directory. Apart from that, sounds good.

You may make use of a method thats frequently known as "hotlink protection" whereby you block demands with HTTP referers that aren't your personal site. mod_rewrite is excellent with this. Here's a good example:

RewriteEngine on
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?yourdomain.com [NC]
RewriteRule \.(jpg|jpeg|png|gif)$ - [NC,F,L]