Dean directs a forum user to modify his doc_root in config.php; apparently, doc_root needs to be the actual web root, not the local root for the textpattern installation.

Gave it a try here, and yes it did the trick! The image tab doesn’t have an error, the organise tab doesn’t give the ”!” warnings, and my stylesheet is still found.

And, I wish I had more time to climb around in rock boots.