I found a work around solution …
I went to “gallery” (the green icon on the left), then “lightbox effects” and try one of the proposals (thickbox and no lightbox do not work , lightbox : the images are complety deformed, all the others work fine).
I deactivated all the plugin I have (Captcha code and Jetpack by WordPress.com) problem still there so I reactivated all the plugins.
I also uninstalled the nextgen plugin and then reinstalled it : problem still there.
