this was originally added by...

commit 67c3e2ab
Author: Caolán McNamara <caolanm@redhat.com>
Date:   Mon May 25 15:00:47 2015 +0100

    impress: right click to insert image

    Change-Id: I521f074bbd500b81d74be7e0e771bd49efc29c9c

then removed with...

commit 7edfee8e
Author: Yousuf Philips <philipz85@hotmail.com>
Date:   Fri Dec 11 18:53:15 2015 +0400

    Rearrangement of various context menus in Impress

    Change-Id: I6674254898dc6d7351ec4450e2aa5ac3f177bdbf
    Reviewed-on: https://gerrit.libreoffice.org/20640Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Yousuf Philips <philipz85@hotmail.com>

but no particular indication as to why

Change-Id: I8b0ec6c22732d347fc7d354dc5ee5e05b5a3ec79