Cannot see logo image

Hi,
When i enter the logo url (from my media library) in the related cell in Mesocolumn options, i cannot see the logo. I tried in jpeg, in png, in différent sizes but only the alt attribute of the image appears.
how can I do?
(I have the same problem with the favicon, although I selected a 16×16 pix *.ico format)
Logo and favicon show correctly if I install another theme.
Many thanks for your help.
Laurence