Picture uload issue

Hi All,
I have got a funny issue. I want to upload pictures to the directory "files". When running the application, picture are actually uploaded to this directory but no thumbnail is shown and neither can I click it (missing picture graphic is shown)
When I hover over the graphic I see the slash "/" behind files www.mysite.com/xxx/files1234.jpg is missing. I tried adding this in the field parameters but this does not work either.
I checked the directory and the file is uploaded as well as the thumbnail, also the field shows the flesname correctly.
Thanks for your support!
Jos