Hi. Installed recently KDE. Wanted to change wallpaper. Opened System Settings, searched for it there. I was very surprised not to find this standard feature there. But was even more surprised to find it finally on left mouse button click. Wallpapers situated in Desktop Settings. But, DS itself IS NOT a part of System Settings menu. It could be reached only by right button mouse click. It is makes no scene. Naturally, it should be in Appearance section. Sure, it is logical to change wallpaper by right mouse click, but not logical not to find it is System Settings. Please, add this section to Appearance.
Since you can configure the Wallpaper on each screen in each activity separately, it needs to be part of the configuration for that containment. Systemsettings has no idea what desktop containments you are using, and can only configure global settings. *** This bug has been marked as a duplicate of bug 155056 ***
I guess 80% of users don`t care about activities, and want to use only one wallpaper on only one screen. There could be a link, or count to existing activities in SS, where you can place wallpaper for each individually. You know, as an new user I spent a lot of time to figure it out (this is my 4th try to start using KDE. Previous 3 I droped because of wallpaper and applets mess in panel (they lost their position accidentally, and there was no obvious way (hold both mouse buttons) to pin it to specific position). You have a beautiful DE, but those small things make it user unfriendly.