If you want to change it reeeally badly, you can do it in the template:

File: /skins/larry/templates/login.html
Line: <roundcube:object name="logo" src="/images/roundcube_logo.png" id="logo" border="0" />

Same goes for classic skin:

<roundcube:object name="logo" src="/images/roundcube_logo.png" id="logo" border="0" style="margin:0 11px" />