Many thanks Ivan. In reading the log file in /tmp I understood the problem that is not an org-mode issue, but a latex one: some parts needed '$$' tags. 
Anyway, thanks for your reply.

Best wishes,

Jo.

Le mer. 22 mai 2019 à 21:46, Ivan Tadeu Ferreira Antunes Filho <ivanaf@mit.edu> a écrit :
I was having the same problem on some latex previews. It seems that imagemagick expects the file to be in the /tmp/ directory while latex itself was putting the file in the same directory.

My workaround was to use 'dvisvgm instead of 'imagemagick.

(setq org-preview-latex-default-process 'dvisvgm)
And when generating latex from a source block, to save the image as svg instead of png.

I think the same can be done for org-latex-create-formula-image-program 

Not a solution, just a simple workaround.

On Wed, May 22, 2019, 15:39 Joseph Vidal-Rosset <joseph.vidal.rosset@gmail.com> wrote:
Hello the list,

I try desperately to export an org file to html with imagemagick option (an not mathjax), and I have this error message:

convert-im6.q16: no images defined `/tmp/orgtexlyqPro.png' @ error/convert.c/ConvertImageCommand/3258.
org-compile-file: File "/tmp/orgtexlyqPro.png" wasn’t produced.  Please adjust ‘imagemagick’ part of ‘org-preview-latex-process-alist’.

I have in my user.el  (= init file)  :

(setq org-latex-create-formula-image-program 'imagemagick)

Any help is welcome,

Best wishes,

Jo.
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