CKEditor doesn't show images

I'm using CKEditor in Drupal 6.15; the interface seems to work fine, but when I try to upload and insert an image (or a swf file), I see a red square with a X in preview, then a broken image icon in the content space (instead the right image).





I can actually use the demo you provide, it just works perfectly even with my uploaded images... so I guess this problem comes from my CK configuration.
Btw, I can copy & paste images from other sites and correctly see them, even resize em!
The thing I can't do is see the images I upload

thx for any tip


edit:
-update-
If I link an external url of an image it works and this makes me think about a permission problem with the drupal/sites folder on the ftp.
I see my uploaded images with CKFinder in the "images" folder under drupal.
But when I select them to insert in the content space, they are shown as in screens...