Yes it is cool to customize OWA but you shoul really read the fine print on the Exchange license before messing around.
OWA 2000
To replace the standard image found at the top left corner you should change the file named logo-ie5.gif found at %programfiles%\exchsrvr\exchweb\img . The files size shuld be 138×48 .
OWA 2003
Since it is more complicated and shiny there are more things to change.
The following article describes the different options:
Skinning Exchange 2003 OWA