This images embed a non free ICC profile that doesn’t allow
modification:
http://web.archive.org/web/20020603153925/srgb.com/usingsrgb.html
Please use the proposed version stripping away this copyrighted binary
profile, probably embedded by an image software at one time:
exiftool -icc_profile"-<=" phpBB/adm/images/phpbb_logo.png
I’ve checked the resulting image and trust it is identical to the
original.
Some more background on the issue:
https://lists.debian.org/debian-devel/2014/05/msg00339.html
PHPBB3-12582
