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 https://www.mysite.it/wp-content/plugins/nextgen-gallery/tinymce3/langs/it.js file points to.