> But now it does not work in my Gnus if I want to include some png images of
> proofs... I have to deleted these "dangerous lines"...

Sorry, I missed this the first time around.  What does gnus have to do
with this?  You were talking about exporting to HTML but never mentioned
gnus in the original post.

If you are trying to write an html based email, you should be looking at
different functionality than exporting.  Check out org-mime-htmlize, for
instance.  Exporting creates files which reference other files (for
images).  Messages are different.