The lock screen & Account picture in PC Settings have a "browse" button. The Start Screen doesn't, so I agree, there doesn't seem to be a way.
If you can find where the start screen pictures are stored in Windows, then maybe you could add or substitute in your new picture and get it to work.
This seems to be a desktop application and so theres no chance to get this working on surface. However, due to the popularity of customizing the several backgrounds, the chance of getting an app doing this (if possible) could be quite god.
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.