Just like Nautilus (see #29970), GNOME Control Center also uses
gnome-desktop for generating thumbnails. In particular, it tries
to make a thumbnail from a file choosen as a profile picture, and
when it does not succeed, it will not allow that file to be chosen.
Of course, whithout a thumbnailer, it will always fail.
43129a1cfd/panels/user-accounts/um-photo-dialog.c (L190-L192)
Since gnome-desktop scans `thumbnailers` directories under the paths
in `XDG_DATA_DIRS`, gdk-pixbuf had to be added to the path to provide
access to image thumbnailer.
a093bf8b88