If you've created an encrypted disk image to keep your sensitive documents in, then there may come a time when you want to change its password. Maybe you drunkenly gave it out to all your friends, even that shifty-eyed dude. Maybe you just want it to be something more secure than "fido123." In any case, it's very easy to do. To get started, open the Disk Utility program (it's in Applications> Utilities), then look within the left-hand column for the disk image in question.
If it looks like the screen shot above, the image is mounted. Since you can't change the password until it's completely ejected, select your image and click the Eject button in the toolbar.
After you've done that, it should look like this:
Then all you have to do is select the image and choose File> Change Password. You've gotta know the original one, though; this isn't something you can use to gain access to an encrypted disk image when you've forgotten the password. Bummer.
Anyway, after you've entered the original password, you'll type in the new one you want to change it to twice, and you're good to go! Be sure to deselect the "Remember password in my keychain" box to make your secure container really, well, secure. You want to have to enter this password every time you need access.
One more thing: If for some reason the image you need to change isn't showing up in Disk Utility, choose File> Change Password with nothing selected from the sidebar, and you'll get the familiar "open" dialog box to allow you to pick your image from the Finder instead. Then you'll follow the rest of the steps I've outlined above.
Simple as can be, right? Now all of your disk image passwords can be more complex than just your pets' names. You guys let me know if I need to scold you into using stronger passwords, OK? I'm prepared for that.