I found a fix that worked for me, it involved simply opening the following folder and deleting the TranscodedWallpaper.jpg file contained within.
%USERPROFILE%\AppData\Roaming\Microsoft\Windows\Themes\
Once I did this, I closed the Personalize screen and reopened it, and then I was able to once again change wallpaper to something considerably less drab than one of the various solid colors.