If you mean change the hovertext, you probably need a custom theme, with the following function:
function logo()
{
$yourtext = 'test';
$this->content['logo'] = preg_replace('/TITLE="[^"]+"/i','title="'.$yourtext.'"',$this->content['logo']);
qa_html_theme_base::logo();
}
If you want to change the image on hover, just use css and hide the logo image on hover:
function head_custom()
{
$this->output('
<style>
.qa-logo-link:hover img {
visibility:hidden;
}
.qa-logo-link:hover {
background-image:url(
http://www.question2answer.org/qa/image/4364140800340146597?s=40);
}
</style>');
qa_html_theme_base::head_custom();
}