Thanks for all of the followups: here are some questions/issues about them:
(1) Len suggested using the Image Magick programs, particularly convert.
Thanks Len for the suggestion.
My problem with convert is that i don't know how to use the -size option,
and there doesn't seem to be a -format option:
convert source_file.jpg -size 80x54 target_file.jpg
doesn't work. It creates a file, but the size, as seen from the identify
program, is much larger. Also, the gimp's thumbnails seem to be in
pnm format, but the file names don't reflect this. (And the -size
option for convert source_file.jpg -size 80x54 target_file.pnm also
doesn't work for me.)
My Image Magick version is 5.4.3.
Otherwise i'd be happy to use this and write a script to automate
the process. Thanks again Len for the suggestion, and i wish i
could follow it.
(2) David suggested that i use the plugin described at
Well, i downloaded it, and it compiles, and i even
managed to get it into the gimp (i think, anyway: i
used File > Preferences, then Directories > Plug-Ins
to add the directory for it, and restarted the gimp).
But i'm so green and clueless i don't know how to get
to the Input page David's web site describes (i don't
know anything about using plugins).
Thanks David for writing the software and gpl'ing it,
putting it on a web page, and pointing it out, and if
i were a little more competent i'd use it, but i don't
know how to use plugins. Anybody who wants to point out
a faq on what must be a trivial operation please do so,
(3) Gail suggested using GQview, which i can invoke by
typing gqview. And indeed, if you select the View > Thumbnails
options, it will generate thumbnails for (at least) all of the
jpg documents in the directory you are in. There is an
Edit > Options menu, which leads to a panel with an image tab
in which you can specify to cache thumbnails.
It does cache the thumbnails, although not in .xvpics but in
some directory ~/.gqview_thmb, and they're not named the way
gimp likes them, but with a .pnm tacked on the end of each.
But it's easy enough to write a script which moves all the
thumbnails into .xvpics and renames them (which i did).
However, if you do, then gimp doesn't recognize them, but
Thumbnail doesn't have the 'P7 332' header.
Thanks again Gail for the suggestion, which wouldn't have
occurred to me, and i wish i could take the final step to
make it work (putting in the P7 332 header).
(4) Roman suggested open image in the directory for the preview,
then `on the image right-button and file, guash'.
I'm not quite sure if this a typo (guash for push?), or just some
other piece of information that i'm not clued in on.
But i don't want to push the button hundreds of times, if it
is push. I would like to do it a whole directory at a time.
Thanks Roman anyway for the suggestion, i appreciate all the help
i can get.
So thanks to all (Len, David, Gail, Roman), and sorry i'm not such
a good student. You are all very helpful teachers though.