Decode given base64-encoded string as a newly inserted 1-column image at the end of the list.
The argument string must have been generated using command uint82base64
G'MIC is an open-source software distributed under the
CeCILL free software licenses (LGPL-like and/or
Copyrights (C) Since July 2008,
David Tschumperlé - GREYC UMR CNRS 6072, Image Team.