I'll assume you're speaking of the picture attached. Typically I'd not advocate digging around in your /System directory, but the picture is located:
/System/Library/LoginPlugins/BezelServices.loginPlugin/Contents/Resources/BezelUI/Images/
and it's titled eject.png. If you replace that file with a different image of the same exact name, it should use your new image the next time you press eject (you don't even need to logout). I'd advise backing up your original, though, so you have something to revert to.
EDIT, Disclaimer: While this really has no negative consequences on the stability of your Macintosh, you still do so at your own risk. The /System directory is not something that you should mess with if you don't know what you're doing.