[Typo3] images won't show up

[Alex]

Hi folks, I'm new to Typo3.
Just installed quickstart-3.7.0...
I'm having problems in getting some images displayed, not only the ones 
I upload, but even nearly all images that comes with the Bigfeet demo 
site.
the only images I can see are the bigfeet logo (bigfeet_logo.gif) and 
the page background (beckback.jpg) which are part of a template.
if I go in the Filelist module and click on an image name, the images 
are there and show up, thumbnail preview works fine too.
but no image is showed in the FE nor in the BE, it seems that just the 
thumbnails work in the BE.

the html source of the wiew item popup shows that no URL is provided to 
it:
<img src="../" width="" height="" border="0" alt="" />

does anyone have an idea of what I could have done wrong?
ciao and thanx in advance
Alex