[BUGFIX] Move default avatar image from background css to img tag

If a transparent avatar image is assigned the default avatar no longer
shines through.

Resolves: #70220
Releases: master
Change-Id: I889c706616876d45d0d9a371b96591ec27aa264c
Reviewed-on: https://review.typo3.org/45191


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>