We did not change anything on your site except loading one image and trying to watermark it.
Generally, you have some problems in your theme (and plugins maybe). From what we noticed:
1. Try to open that link for example http://essai-crea-ct.fr/dwierez/?attachment_id=428 (a link to attachment page) – a fatal error
2. Your theme is overriding jQuery UI styling on Image Watermark settings page. It makes you unable to modify some of the plugin settings.
3. Many different scripts are being added to admin pages, even if it is unnecessary and may cause unexpected conflicts (for example the jquery colorpicker is shipping with wordpress since 3.5 – however your theme is embedding it again).
That how it looks for the first glance. I suggest you change the theme or hire an experienced WordPress developer to clean all that up.