It may have recorded the password in your keychain. You can open Keychain Access, it's in the Utilities folder. Once that's open, go into its preferences and you can check to show keychain access in the menubar. Look for a black padlock with useful options. I like to turn that on with my computers.
There's no need to create a new disk image. You can look through or search keychain access for the entries for your disk images. Delete any you don't want to be there. It will ask for them next time you open a disk image that needs one. There's usually a "remember password in keychain' option in those dialogs, make sure you don't check it when entering your password if you want to keep your passwords out of the keychain.
