Huh? If I've understood the question, it's actually fairly easy -took me about 2 minutes if I recall, but was a couple of months ago so I'm not dead certain on the details.
In my case it was something like:
- Find lockscreen image in C:\Windows\Web\Screen\
- Right Click to 'Set as Desktop Background'
- Right Click on Taskbar > Properties > Navigation Tab
- Tick 'Show my Desktop Background on Start'
...and I now have the same background on Lockscreen, Start Screen, and Desktop Wallpaper.
Might be harder if you want you Desktop wallpaper different, but this works to my liking as it's all consistent.