The problem is the relative URL. You have to be careful with relative URLs in WordPress. Relative URLs are interpretted by the browser but the browser doesn’t know about the URL rewriting that happens behind the scenes.
Think about where your index file appears to be– http://example.com/— vs. where it actually is– http://example.com/wp-content/themes/your-theme-name/index.php. Similar problems occur with image locations. The apparent location– the path relative to the displayed URL– tends to be wrong.
You need to provide an absolute URL for your image src using, probably, get_template_directory_uri() or get_stylesheet_directory_uri() as appropriate.