Because a frame can have multiple (fallback) images, so this would have been
added only to the first one.

Change-Id: I1c41c5e6fd3959fb5cfb5afb5d4fe98b2e5e3c73
Reviewed-on: https://gerrit.libreoffice.org/46243Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>