Free images of praying people are available for download from the website of The Church of Jesus Christ of Latter-Day Saints. It is permitted to use this substantial collection of photographs for personal or church-related, noncommercial use (except for individual images that are labeled otherwise).
The same website also has a selection of video and audio files related to prayer.
Another source for images of people praying is the website of the San Francisco-based Islamic Bulletin, which provides a series of photographs depicting people engaged in prayer. Some of these feature solitary individuals, whereas others show many. Some images on this website show people praying in extreme environments, such as underwater, in the desert and on a snowy mountain.