Google how to change the login screen background. It involves registry tweaking, so I wouldn't mess around with that unless you are completely sure of what you are doing.
If you mean desktop background, right-click and go to Properties/Personalise (Depending on the OS you're using). Go into desktop background settings and select whichever picture you want as your background.
There are tools that change the login screen background for you but the problem is every 2-3 windows updates will revert that registry entry back to default. Because of that I think he is probably talking about the wallpaper.
Unless.. are you talking about a plain black or white screen immediately after hitting the power button to boot the system that says acer? That would be your BIOS. That cannot be changed.
No one mentioned this fix: Click the Settings tile in Start (or under Apps) and select PC and devices. This will bring up the Lock screen page where you can change the picture from pre-selected options or browse your computer for a personal choice.
No one mentioned this fix: Click the Settings tile in Start (or under Apps) and select PC and devices. This will bring up the Lock screen page where you can change the picture from pre-selected options or browse your computer for a personal choice.
Since the original post was from 2013, it is highly unlikely that this is Win 10 or "tiles".