The Photo Gift feature can be enabled/disabled in the settings, by the host. When enabled, guests can select a photo and then select a gift (e.g. mugs, puzzles, blankets and a variety of prints), using the selected photo.
To purchase a Photo Gift, click on the photo you want to use with the gift. Then click the gift icon in the bottom right, as shown below.
The following screen is then displayed to select the desired gift.