Most other DE's and even just regular WM's when used with feh can use a single large wallpaper spanning multiple screens, eg 5760x1080 for triple head. KDE 5 however can not do this and it also blocks feh from working as well. This used to work with KDE 3 and earlier but appears to have broken since then with the rewrite.
I wasn't sure which component was the better fit, Image Wallpaper or Multi-screen support so put it under Image Wallpaper since that is likely where it needs to be fixed.
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Nate's answer: