Logo on Login Screen

Hi,

I’ve noticed when I login to other sites that use WP that some of them have their logo appear above the login box rather than the WP logo.

I’m wondering how I can do this?

I’ve looked in the WP dashboard but can’t see anything so I’m wondering if there’s a plugin that allows you to do it. I guess the other option is that its written into the theme they are using.

Many thanks