This ServiceMenu allows you to set images as Desktop Wallpaper/Background from within Dolphin's context menu. But to save you some disappointments here are some facts:
* Currently only one boring desktop/activity/monitor is supported , for other setups if it works for you, great, if not am sorry. People have reported that it didn't work on such setups.
* Widgets need to be unlocked, otherwise it won't work. Thanks @FraQu, see (1)
* Am neither a developer nor an expert on this area, just a "normal" user. Any Feature requests/Bugs should be directed to KDE bug tracker. See (1) and (2).
* Yes, I know it is s... but hey, if you know a better way, let us know :-)
*Still reading? Nice! Here is how you can use it:
- Copy/move the downloaded file to ~/.local/share/kservices5/ServiceMenus/ (make that directory if it doesn't exist).
- Rightclick an image (png/jpeg) and click "Set as Wallpaper".
(3) dbus command taken from: https://www.kubuntuforums.net/showthread.php?66762-Right-click-wallpaper-changer&p=387392&viewfull=1#post387392
(4) It's also true, more text here than features :-)