I did not changed any setting in css or in multichoice/display.html.
The &&plugin&& is correctly converted in all situations (the image appears then disappears in the initial display (preview or quiz) .
After a submit then everything is dispalyed correctly.
A similar image put in the question text is displayed correctly in all cases.
So there seems to be something (could be related to the option->feedback set after a submit) that either control the display part of the answer (so css related) or the control of the loading of images.
the url conversion for questiontext is done in type/questiontype.php/function print_question()
$question->questiontext = quiz_rewrite_question_urls($question->questiontext, 'pluginfile.php', $context->id, 'question', 'questiontext', array($state->attempt, $state->question), $question->id);
the url conversion for answer is done in multichoice/questiontype.php/function print_question_formulation_and_controls()
$a->text = quiz_rewrite_question_urls($answer->answer, 'pluginfile.php', $context->id, 'question', 'answer',array( $state->attempt ,$state->question ), $answer->id);
I will set a tracker issue on adding image to multichoice answers to continue this quite technical discussion.
Pierre