By chance, I have found new element on the "file on the Czar" by Summers & mangold (a classic one in my litterature ;-))
with other photos, drawings and pictures, is on the end of the book a document intituled "draft of the governor house at tobolsk" and made by people wanting to free romanovs. This document has been saved, said the caption, by Sydney Gibbs. Obvisously, it's not tobolsk house but Ipatiev House and at the place where I supposed they was fireplace (in dinning room and otmaa room), has been drafted a box and a triangle to symbolise the fireplaces. They is also a big rectang in the middle of the supposed dining room. Then I can conclude the mysterious picture is well the dinning room !! Sorry, I haven't scanner so I can't had it but if someone has Summer & Mangold book and a scanner, perharps he could add it here