Change Continue reading with Image in function.php

In my theme function.php it has this line. I dont understand it because I am not a programmer.

$more_tag = theme_get_array_value($args, 'more_tag', __('Continue reading <span class="meta-nav">&rarr;</span>', THEME_NS));

I want to change the wording with an Image. I tried using css and add background image but failed. Can someone help.