Latest github is fine, that's what I'm using.
Well, I'm kind of at a loss. I've tried every combination of directories, prefixes and postfixes, and (with the obvious exception of using the same name in the same folder as the main image) it all works for me.
Have you tried just specifying a subdirectory for the thumbs and not using pre/postfixes? So just specify the thumbs dir as /same/path/to/images/thumbs. I'm interested in whether this is something specific to using pre/postfixes, or if it's a more generic problem with creating the resized image. You said that resizing the main image is working, so theoretically there should be no problem resizing the thumb, as it uses the same routine internally. But ... I'd still like you to test just using a subdirectory.
-- hugh