If you are comfortable with Photoshop, you can use my
Logo Creation Guide here to make a PNG-24 file at 256 x 256 pixels.
Name the file AltLogo.png and put it in the Aquarium's data folder. On XP this is c:\documents and settings\(yourusername)\Application Data\Marine Aquarium 3\
The next time you start the Aquarium (by double-clicking, not as a screen saver), press L to toggle the logo.
Note that the Beta is strictly a test, so you cannot set your logo to appear all the time, only by running the Aquarium manually and pressing L.