The adm/images/phpbb_logo.png image 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"-<=" 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