Hasse Ramlev (in Danish) is the man behind the latest major addition to the K2 core, which allows you to upload pictures to use in your header as well as manipulate the header text-size directly from the new K2 Picture Upload control panel. Grab it from SVN now, or get the latest nightly (which will take a few hours to generate)
Thank you very much to Hasse!
PS: If you already have Hasse’s original plugin installed, deactivate it and relocated the header pictures to /images/headers/ in the K2 folder.
Also, make sure the /images/headers/ is writable.