Phpwebgallery can answer the demand

1/ To be effective it is necessary to put one watermark on all the photos

2/ Put category in private

VDigital wrote:

Category in private will not help to sell anything. ;-)

You should prepare your pictures. Reduce enough and watermark them.
There is absolutly no other solution on the web (PhpWebGallery or any other).

Agreed.

Yes, Reducing them to 72 DPI instead of being high resolution at 300 DPI would go a long way with a watermark, and scaling them down to say 450 px approx. I'm assuming stuff like 300 DPI and image size as this can vary a lot. If it can be resized to 800x 600 without much loss then you have to go even lower on the image your displaying pixel wise and reduce quality.

The secureimages plugin could be used to mask the directory locations of the images.

Personally, I would only have low resolution watermarked images on the server and when he recieves payment for a sale, he sends the client the high resolution image(s) via email. If it takes off large scale then look in to a more convienent way of selling them.

It would have to be mentioned clearly that this is method of delivery. I don't see someone that is willing to buy a image in the first place minding waiting for a email to arrive with the goods.