Have you turned on Windows Spotlight in Windows 10? It's a new Lockscreen background every few days and so far I've been enjoying the pictures tremendously.
If you haven't tried Spotlight before, try turning it on:
In Windows 10 (1511) open the settings panel and navigate to Personalization and then Lock screen. Select Windows spotlight in the dropdown for Background. Scroll all the way down and hit Apply.
Now, whenever you see a picture you love, unlock your machine, hit [win]+r on your keyboard and paste the following location:
This will open up a folder with a bunch of files, each with a random file name. One of these files is your current lockscreen background. What I generally do is that I type cmd in the location bar of explorer en hit enter, this opens a commandline prompt in the current folder.
Then copy all the files to a temporary location and then rename them to *.jpg in one go:
C:\Users\me\AppData\Local\Packages\Microsoft.Windows.ContentDeliveryManager_cw5n1h2txyewy\LocalState\Assets> copy *.* c:\temp\Images\*.jpg
And then you can navigate to that folder, check the image preview in explorer and pick the backgrounds you like. Of most backgrounds you'll find both a portrait and a landscape version.