Most Mac JPEG programs rely on Apple's JPEG implementation, which is part of
the QuickTime system extension; so you need to have QuickTime installed.
To use QuickTime, you need a 68020 or better CPU and you need to be running
System 6.0.7 or later.  (If you're running System 6, you must also install
the 32-bit QuickDraw extension; in later Systems, that is built in.)  The
latest officially released version of QuickTime is 3.0, available from
http://www.apple.com/quicktime/.

QuickTime 3.0 can read progressive JPEGs (but not write them).  Older
versions of QuickTime can't handle them at all, and are also more likely to
crash if fed a corrupted JPEG.  If you're using QuickTime-dependent programs
to handle JPEG then I recommend upgrading to 3.0 pronto.  (Note that many of
the programs recommended in this section contain their own JPEG codecs and
don't depend on QuickTime.)

Mac users should keep in mind that QuickTime's JPEG format, PICT/JPEG, is
not the same as the Usenet-standard JFIF JPEG format.  (See part 1 for
details.)  If you post images on Usenet, make sure they are in JFIF format.
Most of the programs mentioned here can handle either format.

The largest Internet collection of Mac software is the Info-Mac archive,
which is mirrored in many places (the master site is only directly
accessible by the archivists themselves).  The pointers below cite Apple
Computer's mirror site, but you may get better service from a mirror site
closer to you.  See "Introductory Macintosh Frequently Asked Questions" in
the comp.sys.mac.* newsgroups for the current locations of mirrors.

JPEGView is an excellent free program for viewing JFIF,PICT/JPEG,GIF,TIFF,
and other image files.  It can convert between JFIF and PICT/JPEG and can
create preview images for files.  The current version is 3.3.1, available from
http://www.umich.edu/~archive/mac/graphics/graphicsutil/jpegview3.31.sit.hqx.
Requires System 7; QuickTime is optional.  JPEGView is a fine viewer with an
unusual but well-thought-out design (no scroll bars, for example).
Unfortunately, it hasn't been updated in a long time, and is starting to show
its age.  There are reports of bugs under System 7.5.3 and later.  Also, its
built-in JPEG decoder doesn't know about progressive JPEG.  If you like
JPEGView, I suggest installing QuickTime 3.0 and setting JPEGView to use
QuickTime.

Jade is a new, very promising freeware viewer for JPEG, GIF, PICT, and
BMP images.  It's fast, simple to use, and has preview and slideshow
capabilities.  And it supports progressive JPEGs.  Since JPEGView is no
longer being updated, Jade will probably supersede it as the most popular
free Mac JPEG viewer before long.  Current version is 1.2, available from
ftp://mirror.apple.com/mirrors/Info-Mac.Archive/gst/grf/.
Requires 68020 (or higher) or PowerPC, as well as System 7.5 (or later) or
Thread Manager.

GIFConverter, a shareware ($30) image viewer/editor/converter, supports
JFIF,PICT/JPEG,PNG, and many other image formats.  Current release is 2.4.4,
available from http://www.kamit.com/gifconverter/.  Requires System 6.0.5 or
later.  GIFConverter is not better than JPEGView as a plain JPEG/GIF viewer,
but it has much more extensive image manipulation and format conversion
capabilities.  Also, GIFConverter is your best bet if your machine is too
old to run System 7 and/or QuickTime.  Hint: if GIFConverter runs out of
memory while loading a large JPEG, try converting the file to GIF with JPEG
Convert, then viewing the GIF version.

GraphicConverter is another popular viewer/editor/converter.  It has even
more functionality than GIFConverter, but is correspondingly larger.  Great
if you like lots of options.  Shareware, $35.  Current version is 3.6,
available from the author's website http://www.lemkesoft.de/ or various
mirrors.

Sam Bushell has prepared a couple of simple but nicely done drag-and-drop
converter applications, "To JPEG" and "Progressify".  To JPEG converts any
file format understood by QuickTime to regular or progressive JPEG;
Progressify converts losslessly between regular and progressive JPEG
formats.  Both are free and require System 7.0 or later.  Available from
http://www.pobox.com/~jsam/to-jpeg and
http://www.pobox.com/~jsam/progressify.

Cameraid is a useful utility program designed for users of digital cameras,
but having general interest as well.  It does image downloading from many
makes of digicam, lossless rotation and other transformations of JPEGs,
and display of auxiliary information that many digicams include in their
JPEG output files.  It's also a nice viewer.  Version 1.1.1 is available
from http://www.clinet.fi/~jmunkki/cameraid/.  Shareware, $15.

Photoshop 4.0 supports progressive JPEG.  If you have an older version,
you can get two different plugins that enable progressive JPEG support
(they also work in other applications that support Photoshop plugins).
One is ProJPEG, available from ftp://ftp.boxtopsoft.com/pub/
(shareware, $25).  The other is JPEG Transmogrifier's plugin version,
available from http://www.in-touch.com/jpeg.html (shareware, $22).
ProJPEG is worthwhile even with PS 4.0, because it has a nifty preview
of the results of different compression settings.

HINT: You must set the file type code of a downloaded JPEG file to 'JPEG'
to allow Photoshop to recognize it.  Most of the other programs suggested
here are not so picky about file type codes.

HINT: if you use Fetch to retrieve files by FTP, make sure ".jpg" is in its
list of binary file types under Customize/Suffix Mapping.  Otherwise Fetch's
"automatic" retrieval mode will retrieve JPEGs in text mode, thus corrupting
the data.  Old versions of Fetch do not include ".jpg" in the default list.
Also, Fetch 3.0 is buggy; get 3.0.1 or later for reliable uploads.