In C#, you can change the Windows 10 background picture using the SystemParametersInfo function from the user32.dll library. Here's an example: 

uAction: The action to be performed. In this case, the value SPI_SETDESKWALLPAPER is used to set the desktop wallpaper.

uParam: A parameter that depends on the action being performed. In this case, the value 0 is used.

lpvParam: A string that specifies the file path of the image to be used as the desktop wallpaper.

fuWinIni: A set of flags that specify how the system should update the user profile and notify other windows of the change. In this case, the values SPIF_UPDATEINIFILE and SPIF_SENDWININICHANGE are used to update the user profile and notify other windows of the change.

After running this code, the Windows 10 background picture will be changed to the image specified in the lpvParam parameter. You can replace the file path with the path of any other image to change the background picture to a different image.

Note that changing the desktop wallpaper using the SystemParametersInfo function will only change the wallpaper for the current user session. To change the wallpaper for all users, you will need to modify the registry. Additionally, some versions of Windows 10 may restrict the ability to change the desktop wallpaper, especially on corporate or enterprise-managed systems.