Just got an E-mail from the MP3TAG developer, Florian and he wrote:

"yes, I'll support the new PICTURE block with the next release of Mp3tag.
The LYRICS tag is already supported, since it is a simple VorbisComment
entry. Just open the extended tag dialog ('View > Extended tags' or
Alt+T) and add a new LYRICS field.

Kind regards,
Florian"

Thanks Florian for supporting the new FLAC Album Art tags (and already supporting LYRICS) in FLAC files!

Your tagging program is a very useful tool!