OK, I think I found out why it is mot displaying the image. Using Firebug I was able to see the HTML generated on the page:
<img class="starRating rate_1" alt="1" style="cursor pointer;padding:3px;" src="http://webpresence.cz/plugins/fabrik_element/rating/images/star_out.png">
Please take a look at the src.
Now for an image that actually shows the src is different:
<img class="sprocket-lists-image" alt="" src="/images/rocketlauncher/frontpage/roksprocket-lists/rs-list-1.jpg">
Looks like Apache does not want do display files located outside the images folder of the www root.
The images folder has been set up in Media Manager of Joomla.
My question is how the src gets generated and can it be done that the png files used by the Rating plug-in are copied to a subfolder of the images folder and displayed from there?
Thanks!