I have the same problem. It happened after moving a development directory to the live directory, I think. But I have never looked in the Media library before.
for a quick fix you can just comment out the line in that file that reefers to that image.
background: url(img/items.gif) no-repeat bottom left;
by adding a /* */ around it
/*background: url(img/items.gif) no-repeat bottom left;*/
The user is you. TinyMCE is the WYSIWYG editor for WordPress. You are just getting a message saying that there is no image in the location the the http://www.mysite.it/wp-content/plugins/nextgen-gallery/tinymce3/langs/it.js file points to.
I am having the same problem. After I enabled the 404 error page script from http://codex.wordpress.or/Conditional_Tags#Any_Archive_Page I started getting these errors so I don’t know how long they have been happening. I also just upgraded from 2.5 to 2.5.1 and moved my site from a test install to the production install.
