There is no difference between fake and real folder sets. It is just a matter what will you save where.
The real advantage of the folder sets is that it is not possible to determine if, and how many folder sets did you create in Espionage, so if you are forced to unlock Espionage you can pick the folder set you want to reveal and there is no way to determine if you revealed the "real" one or some folder set you created for exactly this "forced to reveal" purpose.
The disk images are by default located inside your (hidden) Library folder. It is less that we want them to be hidden, and more that we make sure users do not delete them by accident. This is when you are protecting a folder from your internal, boot, disk. If you are protecting a folder located on an external disk, you will be asked where to save the disk image. This is because with internal disk we know the folder structure in advance, as it is layed out by the OS X, whereas on the external disk we do not, so we cannot default to some predefined folder.
There is no need and no point in hiding the disk images. They cannot be unlocked without password, and since Espionage 3.6, we are creating fake disk images during initial setup, so the number of disk images mean nothing, it is not related to the number of protected folders. So somebody seeing 5 disk images, cannot force you to show him the content of all 5, because there is no way to determine if they are fake or real and you can simply claim that they are fake.
It is simple yet interesting concept and makes the whole Espionage thing very safe.
If you have any further questions, feel free to ask,
Rgds
Zsolt