Usually you have your web server serve those directly from the file system. That is, Struts2 isn't doing it itself.
If you supply which one you are using, maybe we can help configure it.
For example, if you are using HTTPD (Apache) web server in front of your Tomcat7, there will be a folder from which HTTPD serves up files. Perhaps /var/www/html is that location for some system. Then you would just put your static content in /var/www/html/assets.
Edit
If the static resources are to be in the war file, you just build the war and put those files in the root of the war file or in some otherwise-unused folder structure beginning in the root of the war.
For example, if you want to reference an image at http://mysite.com/assets/my.png then you put it my.png in the folder /assets in the root of the war file.
From any HTML page in the site, you would refer to that image file this way:
<img src="/assets/m.png" .../>
This uses the default path on the same web site.