Hack? wp-logo-vs.gif replaced.

Last night, in the admin panel, the WordPress logo showed up in the upper left hand corner like it always does. This morning, I load the page, and that WordPress logo is replaced by an image of a woman’s face.

This happened on one blog only out of six.

Does anyone else know anything about a hack involving this? I’m still going through the site to see what else, if anything, has been changed. And I’d like to know how the hackers got in and changed this as well as what I need to do to block them from getting in again.