Did you check out the codex? http://codex.wordpress.org/Customizing_the_Login_Form
What you are refering to is because of the default title attribute on the logo image. You can change that by adding some code to the functions.php file for your theme. Here’s how (from the codex):
To change the link values so the logo links to your WordPress site,
use the following WordPress hooks example; edit it and paste it below
the previous in the functions.php:function my_login_logo_url_title() { return 'What you want instead of Powered by WordPress'; } add_filter( 'login_headertitle', 'my_login_logo_url_title' );
You can also change what url the image links to with a similar function. See the link to the codex, above for details about that.