I am running OpenSuSE Tumbleweed in an x86_64 with the following plasma packages:
plasma5-workspace-5.3.2-1.1.x86_64
plasma5-desktop-5.3.2-2.1.x86_64
plasma-framework-private-5.12.0-1.1.x86_64
plasma5-workspace-branding-openSUSE-13.2-13.1.x86_64
plasma-framework-components-5.12.0-1.1.x86_64
plasma-addons-akonadi-4.14.3-3.5.x86_64
plasma5-desktop-branding-openSUSE-13.2-13.1.x86_64
plasma-nm5-pptp-5.3.2-1.1.x86_64
plasma5-session-5.3.2-2.1.noarch
plasma-nm5-openconnect-5.3.2-1.1.x86_64
plasma5-addons-5.3.2-2.1.x86_64
plasma-addons-lancelot-4.14.3-3.5.x86_64
patterns-openSUSE-kde_plasma-20150603-5.1.x86_64
plasma-addons-marble-4.14.3-3.5.x86_64
plasma-nm5-vpnc-5.3.2-1.1.x86_64
python-kde4-plasma-4.14.3-3.3.x86_64
plasma-nm5-openvpn-5.3.2-1.1.x86_64
plasma5-workspace-libs-5.3.2-1.1.x86_64
plasma5-addons-kimpanel-5.3.2-2.1.x86_64
plasma-nm5-5.3.2-1.1.x86_64
plasma-addons-kimpanel-4.14.3-3.5.x86_64
kdebase4-workspace-plasma-calendar-4.11.21-2.1.x86_64
plasma-framework-5.12.0-1.1.x86_64

These are current from the Tumbleweed repositories as of a few minutes ago.

I have the desktop wallpaper set to a slideshow with "scaled, keep proportions". The odd problem is that some reasonable fraction of the images are not properly rotated. However, if I right click on the wallpaper and "open wallpaper image", gwenview shows the image properly rotated. Is this a bug (seems likely), or is there a setting somewhere that I missed?

Thanks.

gwenview interprets the exiv2 metadata in the image and rotates the image accordingly.

The desktop will just open the image and display it as it is, ignoring all metadata. The reason is (likely) the different scope: the desktop paints wallpapers, ie. usually data pre-processed for desktop usage, not photos as they leave a camera. So there's no strong point in requiring libexiv2 just for a desktop shell.

There is the: Bug 240198 - Rotate wallpaper images using exif data - https://bugs.kde.org/show_bug.cgi?id=240198