I found that an image is not shown on the front page when you are not logged in. This happens if you upload the picture with the editor and set an image tag into the top section. If you also add a label into the top section and then add the image tag into this label then the image will be shown.
You can see the problem on my test page ... sorry, but it's rather slow. If everything would be correct the you should see the image twice. If you are not logged in you will see only the second image. It's the same image which I uploaded only once.
The first image url is http://moodle4mac.dyndns.org/moodle20/pluginfile.php/2/course/section/64/moodle-mosaic.png
and the second image url is http://moodle4mac.dyndns.org/moodle20/pluginfile.php/44/mod_label/intro/moodle-mosaic.png