read the link you posted back on post #2, do you have to pick a size for the disk image(folder)? what if you dont know how much space you will need if its a folder you will always be adding to?
Yes, you do have to pick a size to begin with- and it is fixed.
But its not that big a deal in practice- when you're getting to your limit you can
-make another one with the same password (if you want),
-transfer your data into the new one,
-delete the old one and
-rename the new to what you called the old one.
This effectively lets you increase the size of the disk image.
Its sounds more clunky than it is to do- and it has the advantage of behaving like the old one.
If I can give you an example: I'm pretty secretive with my schoolwork and keep my work in a disk image. So I can get to it easily, I make aliases to files and folders and put the aliases in my Documents folder. So when I look in Documents i see an alias to, say, "Essay". When i click Essay to open it, the disk image mounts and asks me for my password then opens the file normally. I like this as I can easily see and get to the things inside but know they're safely under lock and key.
This also works if you reference files in the disk image with another program (can't imagine why, but say if you keep your iTunes music in a disk image and want to play a track, it'll ask you for the password to begin with and afterwards not bother you again for when you want to play other tracks in your library). The great thing is, if you increase the size of the disk image as above it'll be act just like the smaller image- the only change being how much space you've got inside.
Hope this helps spread the love with disk images! I was sceptical until I started playing around and saw how useful they can be