Can I steal Windows 10 lock screen images to use as screen background?

Yes you can

Because they are stored on your computer. The windows lock screen images are held in a folder:
%userprofile%\AppData\Local\Packages\Microsoft.Windows.ContentDeliveryManager_cw5n1h2txyewy\LocalState\Assets
If you open this folder you'll find a number of files with no file extension. Some of them are .jpg files and others not. You'll need to copy the folder contents and rename them to find the files you want.
The easiest way to do this is to copy the entire folder 'assets' and paste it to a new, easy-to-find location. Your desktop for example.

• Either back up one level or open %userprofile%\AppData\Local\Packages\Microsoft.Windows.ContentDeliveryManager_cw5n1h2txyewy\LocalState
• Right click the folder Assets and choose Copy. Then right click on a blank area of your desktop and choose Paste.
• Go into the folder you just created and shift right click a blank area. Select the option 'Open a command window here'.
• In the command window type ren *.* *.jpg this will change all the files to a jpg extension.

You can now look at all the files and see the contents in a preview pane. (If you don't have a preview pane you can find it under the View tab of the file window). Those files which don't show an image - delete. You'll find two types of image. Full screen landscape ones suitable for laptops and portrait images suitable for mobile phones and tablets.

Downloaded Office files - Removing Microsoft's safety pin

Periodically I download from my bank a spreadsheet of my transactions. Recently though an update to Excel (version 16.0 as in Office 365) stopped these .XLS files from opening. Instead I see a blank page. To view the contents I must now right click the downloaded file, select 'Properties' and in the General tab check the 'Unblock' at the bottom.  It's a pain, especially since Excel will still warn me that the file came from an external source and still ask me if I want to enable editing. It seems Microsoft use belt, braces (that's 'suspenders' in the US) and now a safety pin also.

I looked for a way round this and in an answer to an apparently unrelated question about slow saving of large Word files, I found a possible solution using a registry tweak. It worked and I now no longer have to unblock Excel files from an external source. Obviously, you should only do this with files from sources you trust (can I trust my bank?).

Note: The method to fix the issue will modify the Registry. Before you do this, make sure to back it up and understand how to restore the registry if a problem occurs. Please refer to this link: https://support.microsoft.com/en-us/kb/322756 How to back up and restore the registry in Windows.

Using Regedit, go to this registry key address:
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Excel\Security\FileValidation
If this doesn't appear to be present, you can create it. If your version of Excel is 15.0 or earlier - just change that to the appropriate number.
Double click the REG_DWORD: EnableOnLoad, set its value to 0 (zero) and restart the PC.

You'll now find you only get two warnings about external files instead of three.

Incidentally this same registry tweak will also help with slow loading and saving of large Office files.