JPG image would have to be JPG + PNG fallback, at which point it's
easier to just write PNG.

Change-Id: I6e4f0759244d29735fdcda8e050d198f118c737d
Reviewed-on: https://gerrit.libreoffice.org/51151Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>