how do i change the background in 17.10

What desktop session are you using, please?

It used to be called 'wallpaper', but now seems to be labeled 'background' when I go in from the startup screen. I always shut my computers right down - ie no power or even connection by wire - when I leave them. I know paranoid, but there you go.
r

You didn't answer my question

your english seems to be of a different form to mine.
the ip used is ubuntu 17.10, or do you mean something else - i'm not sure, does desktop session have another mraning.
i want to alter the opening picture.does 17.10 allow me to?
rk.

The question was, which desktop environment you are using (Gnome, Xfce, Budgie, Unity, LXDE, KDE Plasma, MATE, ...) because the instructions for changing the wallpaper differ for these.

sorry about my confusion ( to me a session is a communication between two or more), i can only assume that my session on ubuntu is on gnome, sine i have not inokes any of the others. thanks for your explanation of the terminoogy.
rk

Try a right click on a free space of the desk top. If you get a pop-up that allows setting the wallpaper, then use this.

Or click on the icon in the top right corner of your desktop for a pull-down box and click on the settings icon (screwdriver and wrench) in the bottom left corner of that pop-op
This should start a system setup window with (among others) an item for changing the wallpaper.

Maybe looking at https://www.youtube.com/watch?v=u7dH7HInP1s helps