...when creating a disc image that you UN-CHECK the add to keychain box when creating your password. This will enusre that no one has access to your pass word and therefore your files.
If you add a password to your keychain, then that means that
only your user account can access and use that password.
If you are sharing a user account with someone else, then you've got bigger security problems than needing to put files in an encrypted disk image.
User accounts are not meant to be shared, and adding a password to your keychain is
never a security risk, with the exception of the situation where you're letting someone else use your user account -- which is silly, counter-intuitive to the whole "multi-user" setup that is Mac OS X, and (pardon my being blunt), just downright stupid.
You could, of course, pick a good password for your user account, store your files inside your home folder (not inside an encrypted disk image),
quit sharing your user account with other users, and achieve the exact same level of security as making an encrypted disk image -- albeit with
much less effort and hassle.
(try only to use 1 master password for you important files, this will avoid mixups!).
Also a very, very stupid way to achieve security. If that one password is compromised, then
everything you've protected with that password has been compromised. Always, always, always, always use different passwords everywhere. Then, store them in the keychain with a secure note, and
quit sharing your user account with other users. You don't share underwear with them -- don't share your other private things as well.
