I wrote a brief article about changing logos in 1.7RC4 the other day. While it may not entirely translate into 1.6ST it should be pretty close.
Its located at: http://www.tmib.net/how-change-osticket-v17rc4-logos(http://www.tmib.net/how-change-osticket-v17rc4-logos)
Enjoy!
note: the primary difference would be the file names of the logos in 1.6.