There are filename mismatches between the .desktop and .jpg files of two of the F28 supplemental desktop backgrounds, morning-at-étang-de-berre and stora-hornsjön; the .jpg includes the accents and the .desktop files (for KDE) and the XML files (for GNOME and MATE) do not:
This causes the .desktop files not to be installed, as can be seen in the build.log (look for "cannot stat"):
The result of all these mismatches is that these two backgrounds are not made available in any of the three affected desktops.
Proposed as a Freeze Exception for 28-final by Fedora user yselkowitz using the blocker tracking app because:
Two of the supplemental backgrounds/wallpapers are not selectable through the usual means on the GNOME, KDE, and MATE desktops. IIUC supplemental artwork does not block the release, but perhaps a freeze exception may be warranted here.
f28-backgrounds-28.1.3-1.fc28 has been submitted as an update to Fedora 28. https://bodhi.fedoraproject.org/updates/FEDORA-2018-292ca32b20
Discussed during the 2018-04-16 blocker review meeting: 
The decision to classify this bug as an AcceptedFreezeException was made:
"This affects use of the affected backgrounds on at least the MATE and probably the Cinnamon live images, so it's worth fixing in the frozen package set"
f28-backgrounds-28.1.3-1.fc28 has been pushed to the Fedora 28 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2018-292ca32b20
f28-backgrounds-28.1.3-1.fc28 has been pushed to the Fedora 28 stable repository. If problems still persist, please make note of it in this bug report.