I have just noticed that
Tools -> Add Current Album Art to MKV File doesn't work any more. It's not due to a bug in my code, but to a broken mkvpropedit.exe. In the latest version of BD3D2MK3D, I have included mkvmerge and mkvpropedit taken from v8.2.0 of MkvToolnix. Unfortunately,
mkvpropedit v8.2 is broken and cannot add attachments to MKV files any more.
I will not release a new version of BD3D2MK3D just for that, but if you want to add cover art attachments to an existing MKV file with BD3D2MK3D, you should replace mkvpropedit.exe in the toolset directory with an older version. I don't know exactly when that bug has been introduced. Perhaps with v8.0? Anyway, I have tried v7.6.0 (because I have it here), and it works perfectly. So, if you can't get a working version of mkvpropedit, you can download
mkvpropedit_v7.6.0.7z and extract the exe in your toolset directory.
Of course, I have posted a bug report at the MkvToolnix bugs tracker, and hopefully the next version of mkvpropedit will work fine. I will release an update of BD3D2MK3D at that time.
Note that adding cover art to a BD3D2MK3D project should work fine, because the attachments are added with mkvmerge, at mux time. Mkvpropedit is only necessary to add attachments to an existing MKV file (and is only used by the Cover Art option of the Tools menu).
[EDIT] After a contact with Moritz Bunkus (the author of MkvToolnix), it appears that the bug is in a recent version of a library used by mkvtoolnix. But it is sufficient to copy a magic file in the toolset directory and the latest version of mkvpropedit will work fine. So, you don't need to replace mkvpropedit with an outdated version. Just download the
"magic.mgc" file, and place it in the sub-directory "data" in the toolset directory. So, you must have this:
BD3D2MK3D_installation_path\toolset\data\magic.mgc
I have checked here and indeed that works fine.
If you have installed the whole MkvToolnix package, you can also simply copy the data directory from your installation of MkvToolnix to the toolset directory.
Of course, the next update will include that magic file directly in the BD3D2MK3D distribution.