I am trying to figure out a way to have a pictures or pictures work with the screen saver option in windows 10? Currently the picture shows half cut off on one monitor only. I tested with the blank setting and all monitors turned blank so I know that it is working on all 3. Does anyone have a solution for having a picture spanning all monitors or separate pictures on each monitor as a screensaver in windows 10?
UPDATE October, 2017: Windows 10 now natively supports screen savers on all monitors by default. I do not know what version this was implemented on, but I am running 1703 (Creators Update) and this is now the standard behavior for screensavers.
Original answer: The latest information I can find is that the screensavers in Windows 10 only support the primary monitor. Third party tools may allow for multi-monitor support, but it is not yet built-in.
Going back to the release of Windows 10 Microsoft was aware a few people wanted this capability, but not very many, and so they appear not to have made an effort to add this.
I'd suggest you open the Feedback app in Windows 10 and suggest this, or find if someone else has suggested it, and then try to get others to vote for it to raise its visibility and to let Microsoft know it's something people want.
Here's a discussion on the Microsoft support forums on this topic. It began in 2015, and has updates from 2016 as well: https://answers.microsoft.com/en-us/windows/forum/windows_10-start/no-multi-monitor-screensaver-support-in-windows-10/0ed8a245-97f8-4962-9caa-0776d9a8c4f2
MultiPhotoQuotes: Not open source but free (donationware), and designed specifically to show photos across multiple monitors: http://www.donationcoder.com/Software/Mouser/mpq/ (I am the author).