In your product base, you should have a free download product. When you add the free download to a price list, it adds a download icon above the photo when viewing and under the thumbnails (if icons enabled).
How to add a free download.
In Photo Products -> Product Base, there is a Free Download product (this is not added to the sample price list by default).
You will need to add that product to a price list and assign that price list to the page / gallery you want to offer free downloads in. That price list can have other products in it or just the free download.
When it is the only product in the price list, it won't show to section to the right of the photo where products are normally shown.
Free Download Options
When editing the free download in the Product Base section.
This will be the size of the longest side. Enter 0 for the free download to be the largest size available (original).
Add a watermark
Add the watermark you have set in Settings -> Watermarking.
Add a logo
Add the logo you have set in Settings -> Watermarking.
Require the person to be logged in
This option means when they click on the download icon, they are asked to to log in or create an account.
Allow Download All
When selecting this option it will allow the customer to download all the photos in a gallery in a zip file or zip files. They will have the option to do so when viewing the gallery of thumbnails.
Limit the amount of free downloads
This option set the limit to how many photos they can download for free per gallery.
Assign a Customer To Download For Free On A Per Gallery Basis
Starting with Sytist version 1.4, you can assign customers to download for free from certain galleries or sub galleries.
To assign, view the gallery in the admin and click the Photos tab for the gallery.
Click the Free Downloads button.
There you can select the customer and the free download product.
If you only want them to download from a specific sub gallery, view the sub gallery first, then click the Free Downloads button.