Does an encrypted disk image make sense?
* user logins and passwords
Well, you could protect your whole Home Folder...
The thing that worries me about bukling items in a disk image is I believe all your eggs are in one basket. Others feel free to correct me but I believe putting 10 files in one disk image is basically making a single big file out of them. Should that file become corrupted then everything is blocked. Let's say you had a 10MB disk and 10 one MB files. Let's say a 1kB error appears on the drive. With 10 files one of them that uses that spot becomes corrupted but the other 9 are still okay. With a single 10MB file the whole thing includes that error area and as a result the whole file is perceived as bad.
Some individual applications have encryption, though the security level on some has been questioned.
Apple used to have an encryption tool back in pre OSX days but alas it is no more. According to http://osxdaily.com/2007/03/07/ask-os-x-daily-how-can-i-password-protect-a-file/ the zip utility in Terminal can encrypt an individual file. However, most people find Terminal a bit cumbersome. I suppose it would be possible to interface this as a more friendly utility via Automator or and Applescript...
You need to have a good backup plan when using encryption.
Take a large flash drive alone. Copy disk images to it. Should be safe since your data is encryped. Be sure to use a secure password. Write the passward done & leave the paper at home.
Use the sentence password methed. First letter of each word. Example:
Write the passward done & leave the paper at home.