Hi, Thank you for your question and welcome my name is ***** ***** I will assist you. I will answer your questions in turn:
(1) Because a photo has been published on the internet that does not mean it is "in the public domain and therefore free to use". The copyright owner can still prevent people re publishing for their own commercial gain. You would have to individually verify each image is not subject to copyright ownership or obtain consent from the owner;
(2) In the event of the Co-Authors death - the estate of the Co-Author will inherit ownership of the works. Once both authors have passed away the copy right should last for another 70 year depending on which country you enforce it in.
Kind regards AJ