I'm using Imagemagick to create thumbnails for a large directory tree. The only thing I can't see is how to get it to write the thumbnails to a "thumbs" subdirectory!

Either of these two commands from the Imagemagick site does most of the job:

which gives me this error
or this one:
which gives me this error:
Either way, that thumbs subdirectory is always assumed to be relative to the dir from which the command was executed rather than the subdir being processed. It'll process and rename files in the top level dir just fine.

How do I tell it to look for "thumbs" under the subdir it's processing? ?? I've studied the man pages, tried googling everything I could think of, tried variants I invented (that was the most hopeless of all!) and gotten nowhere.

I did manually make all those "thumbs" subdirectories. Ideally, I'd also like to be able to tell it to make that directory on the fly, but that's my next problem, unless someone has the answer.