base_url() should be used for absolute URLs.
If you include ../ you are stating that the URL is relative.
I think your code should be something like this:
<a href="<?=base_url()?>includes/images/logo_fosss.png">
or like this:
<a href="<?=base_url()?>/includes/images/logo_fosss.png">
Make sure your includes folder is in the root folder of your Codeigniter install.
For more information, check out this article:
http://www.webdevelopersnotes.com/design/relative_and_absolute_urls.php3