How extend a wallpaper through multiple screens in 18.04?

Settings for the positioning of the wallpaper are not exposed to the user in Gnome Shell (sigh...). The setting is however available in Gnome Tweak. Gnome Tweak is not installed by default in Ubuntu 18.04 (and 17.10), so you may need to install it first. On the "Appearance" tab, you will find a "Adjustment" dropdown under "background". To extend your wallpaper across screens, select the option "Stretched".

For more advanced users: the setting may also be enabled with dconf-editor, which also is not installed by default. A third way is to change the setting through the terminal, e.g.:

gsettings set org.gnome.desktop.background picture-options 'spanned'

You can reset to the factory default:

gsettings reset org.gnome.desktop.background picture-options