Microsoft puts a lot of effort into finding - and purchasing, usually from Getty Images - suitable images for background (as in lockscreen or desktop) use, so it's understandable that some restrictions should be put in place. But there's no intention to prevent Windows phone users from also using the same wallpaper files. And it's really not too tricky.
1. Install the wallpaper theme pack you want on your Windows 10 (desktop/laptop/hybrid) PC
If you're not sure what's available, then in Store on your PC, scroll down on its home pane until you see 'Windows Themes'. There's a lot in there, so just pick something you fancy and then 'Install' it. (After installation, you can optionally set the wallpaper pack as your Desktop theme in Settings in the usual way. If you're not sure how to do this then just 'Apply' the pack from Store and you'll be taken to the right dialog in Settings.)
2. Find the wallpaper JPG images
This is the only real technical challenge, as it involves delving deep down into a folder structure in Windows Explorer (or any other file manager of your choice on Windows 10). You'll find the new wallpaper images under:
'This PC > Windows (C:) > Users > [your name] > AppData > Local > Microsoft > Windows > Themes > [name of wallpaper pack]'
Which sounds dauntingly long, but it's only a few mouse clicks and you'll soon be staring at thumbnails of the wallpaper images.
3. Copy one or more of the images to your Windows 10 phone
How you do this is up to you and depends on how you normally shuffle content across. I covered this for Macs here a couple of days ago, but you can equally well drag and drop in Windows Explorer directly onto a connected Windows phone. Or you can go via OneDrive, just as easy and doesn't involve wires. In each case, just put the wallpaper JPG files somewhere you'll easily find again, such as the 'Pictures' folder on the internal drive of the phone.
4. Set one of the images as your lockscreen wallpaper or Start background
There are several ways into this. The easiest is probably to go into Photos and refresh (if needed), to see the new image(s). With one up on screen, you can use the '...' pop-up menu to 'Set as' and then choose either 'Set as lock screen' or 'Set as background' (and then repeat if you want both!)
Easy, eh? So the next time you see a tasty Windows 10 Desktop wallpaper pack floated on the Interwebs, you don't have to restrict it to just your Windows Desktop, you can have the same (usually) AMOLED-friendly masterpieces on your Windows 10 phone too.