That's not a standard WordPress file. Looking at how it's set up there (0 width and height so it's not seen) and that it uses a relative URL (which WordPress pretty much ever does) it looks like your site has been hacked in some way to add that in there.
As the standard help goes for this situation:
If your site has been hacked, you need to start working your way through these resources:
http://codex.wordpress.org/FAQ_My_site_was_hacked
http://wordpress.org/support/topic/268083#post-1065779
http://smackdown.blogsblogsblogs.com/2008/06/24/how-to-completely-clean-your-hacked-wordpress-installation/
http://ottopress.com/2009/hacked-wordpress-backdoors/
Anything less will probably result in the hacker walking straight back into your site again.
Additional Resources:
Hardening WordPress
http://www.unmaskparasites.com/
http://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html