Dear Eric, Seb, Achim, many thanks, it works now !
Here is my configuration, at the very beginning of my init.el file :
(message "* --[ Loading my Emacs init file ]--")
(add-to-list 'load-path "~/.emacs.d/org-8.2/lisp")
(add-to-list 'load-path "~/.emacs.d/org-8.2/contrib/lisp" t)
(require 'org-loaddefs)
;(package-initialize)
(global-set-key "\C-cl" 'org-store-link)
(global-set-key "\C-cc" 'org-capture)
(global-set-key "\C-ca" 'org-agenda)
(global-set-key "\C-cb" 'org-iswitchb)
(require 'ox-latex)
(require 'org-mime)
(setq org-mime-library 'mml)
I have succeeded to get an email wih png images which correctly appear in Thunderbird and a link on png images in gnus . Wonderful !!!
Now, the next ideal step would be to be able to use any latex package to get png images via gnus, for example fitch or bussproofs or qtree...
it is not an hurry, I will follow your advice.
Hope that my email will help other beginners.
Many many thanks to all of you !
Jo.