Image files posted on the alt.binaries.pictures.* newsgroups are usually
"uuencoded".  Uuencoding converts binary image data into text that can
safely be posted.  Most posters also divide large posts into multiple parts,
since some news software can't cope with big articles.  Before your viewer
will recognize the image, you must combine the parts into one file and run
the text through a uudecode program.  (This is all true for GIF as well as
JPEG, by the way.)  There are programs available to automate this process.

For more info see the alt.binaries.pictures FAQ, which is available from
http://www.faqs.org/faqs/pictures-faq/
(see also "[24] Where are FAQ lists archived?").