Moved to General discussion as it is a general question.
The easiest way would be to just set the iMac to
auto login to a specific user account, one with no other information and ensuring any other account on the Mac does not allow sharing of folders with other users.
Any reboots would just give access to that account, only users who know the admin name and password would be able to override the auto login.
You could leave Safari running with the album page shown when closing down and have it reopen on starting again. But that wouldn’t restrict what users can do, like Close Safari or visit other sites and download files.
You could try nobkiosk from noblic.com.
Never used any kiosk software so can’t say much more about it.