GEM-View (shareware, $26) displays JPEG, GIF, and other image formats.
FTP from atari.archive.umich.edu:/atari/Graphics/Gemview/gview248.lzh.
This is a well regarded viewer.  The English documentation tends to be a
few versions behind, though.

MGIF is a good free viewer/editor for JPEG and many other image formats.
It's particularly good on monochrome monitors, where it manages to achieve
four-level gray-scale effect by flickering; but it works on all Ataris.
Version 5.00 is at ftp://atari.archive.umich.edu/atari/Graphics/.

1stGuide is a small, fast viewer for all ST/TT/Falcon systems; it supports
JPEG, PNG, and other file formats.  Shareware, $35.  Available from
ftp.cs.tu-berlin.de:/pub/atari/View/1st-guide/1stguide.zip.

The free IJG JPEG software is available compiled for Atari ST/TT/etc
from ftp.cs.tu-berlin.de:/pub/atari/Graphic/jpg6btos.zip.
These programs convert JPEG to/from PPM, BMP, Targa formats.