Oh thats an easy one : Make a .reg file and run it
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Explorer]
"UseDefaultTile"=dword:00000001
Now u go to C:\ProgramData\Microsoft\User Account Pictures
Photoshop the pics to whatever u like
and next time you restart you will have your pic turn into to whatever you added into it
Your done!!!
OPTIONAL:
To remove that annoying microsoft sign in pic
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\System]
"DisableLogonBackgroundImage"=dword:00000001
No phone style swipe lockscreen reg
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Personalization]
"NoLockScreen"=dword:00000001
I keep my regs for every time i do clean install