Yes, the THM management code needs a complete rewrite I'd say. It should basically be handled like cached thumbnails under .jalbum/.cache. You can try the current beta core now, it should at least not give any nullpointer exceptions.
Yes, the messages vanish. As before, no crashes or stack traces - it was just a couple of lines in the console, as reported in my first post.
I was in the midst of adding PDF support to Atom, and ran into a bizarre, related error - the jAlbum core was consistently misreporting the thumbWidth and thumbHeight for a THM attached to a PDF, but only if the fixed shape filter had been invoked. Puzzled over it for a while, then checked the code in my other skins. Hey, presto, this is an old jAlbum bug that I had forgotten about completely - those other skins clearly showed my workaround. I had simply forgotten about it, since I hadn't dealt with it in a while. One more thing to add to the THM Terrors™ list.
The THM stuff has been a minefield forever, and most of us have worked around its problems in our skins, even though we sometimes forget that we have done so! It's certainly a clunky mechanism for users, but anyone who's using it for a bunch of PDF's, for example, is used to it by now.
The integrated browser is a much more visible problem.
And I'd rather have core-generated folder thumbnails than a new routine for THM's.