Clarify license terms for Romain Guy's artwork.

As discussed with Romain Guy in the email thread, the Apache 2.0
license applies to the affected .jpg files now and retroactively from
the time of import.

'wallpaper_goldengate.jpg' and 'wallpaper_goldengate_scale.jpg' were
altered using the exiftool:

exiftool -WebStatement=http://www.apache.org/licenses/LICENSE-2.0

Each of the other images was altered using the exiftool:

exiftool -URL=http://www.apache.org/licenses/LICENSE-2.0 \
    -WebStatement=http://www.apache.org/licenses/LICENSE-2.0 \
    -UsageTerms='Apache License Version 2.0'

Change-Id: I52a032fc6aa6373d2fb0ea941e041ea72764268c