First off, with Lodoss and many other series, Yuuki
himself has published volumes which show each of the
gengas he drew. 

With the drawing I got, this is what was written on the
"Letter of Authenticity" I asked to have printed:
'The gengas were purchased in cut bags and Yuuki's name
as "Genga Man" was on this bag and the cut bag was the
correct bag for these "Genga" - So these are Yuuki's
artwork'

Oh and Art-Toons has no website... for now ^^

Best,
-Jocey