Unable to Upload a logo

Hello,
I am not able to upload a logo.
Regular customer of wordpress, I have no problem to upload a jpg. files in my library.
But in Theme-settings/Header/Logo/insert-your-logo_Upload-or-insert-url,
when I select an image and type on insert in article, I see no effect, neither in my backoffice or on my homepage.
Thank you
Maarten
site : http://www.lememeenmieux.fr/