I see an "accordion example" on page 144 (chapter 4). I work on something similar, I want to add an accordion on drupal forum.

Now I want to see an image when someone click on accordion container (collapsed) and another image when accordion is expanded. A good example is Vbulletin forum, the arrow in forum container.

As beginner I don't know how to implement this feature, I need to check something before image is changed ?
Please help me a bit! Thanks!