The images show each possible answer's "correctness":
Green tick - correct answer
Amber tick - partially correct answer
Red cross - wrong answer
Each image will have the appropriate alt text.
The answer that the student has picked is highlited by changing the background colour: light green, light amber or light yellow, depending on the correctness of the chosen answer. In addition to this, the image for that answer is much bigger in size than for the other possible answers. The alt text for this image will indicate that this answer was picked.
This way, the visual cues, the relative importance of each answer (chosen/non-chosen), and the relative relationships (correct/wrong) are kept. The chosen answer is emphasised by the background highlighting, by the bigger size of the icon, and by the alt text as well. I think this covers all the bases.
While implementing this, I removed the ~100 lines of very similar code from the top of each question type's editquestion.html template, and moved this to a the function print_question_form_start() in the base class, which uses the template question/type/editquestionstart.html. This is described in more detail here: http://moodle.org/mod/forum/discuss.php?d=51590, and is why this patch makes the codebase 523 lines smaller.
jamiesensei