Hi Neverbe! I think you can open the .img file but not sure how. Certainly, it is not an encrypted (secure) file. A user called Wicknix creates the 3MX .img file, so he may step in here and tells us

I think it is mentioned elsewhere on the forum but cannot remember where....
Yes, the install process is the same whether it is the original OS or 3MX or Debian Etch or the Retro version :
- put the .img or .rec file on an SD card
- switch the minibook on
- immediately hold down key F3 and continue to hold that key down
3MX is an OS like Windows XP is an OS. The original OS is a different OS, like Windows Vista is an OS. There are different 3MX OS - versions 1.1, v.2.0, v2.1, v.3.1 and v.Retro. You should only consider installing v.1.1, v.3.1 or Retro as others have problems, for example they are slow or major things do not work.
When do you type
startx ? On certain 3MX OS versions the minibook will boot up to a command line - it will be quite obvious and the text will tell you to type
startx You need only type
startx once, but if you reboot or shut the minibook down, you must type
startx when you boot the machine up again. This is why most people prefer v.1.1 or 3.1 or Retro as you do not have to type
startx - it is automatically done for you.
Neverbe, it is all quite obvious what to do when you have done it the first time.... you cannot break anything

Enjoy!
Unh.