Here is how i did it :

(Be sure you have rootsh & easy-chroot)
Everything typed in terminal is blue
- Put the bt5.img anywhere (i did it in /home/user/Mydocs/bt5, so rest of the howto is based on that but can be changed)
- open Terminal
- sudo gainroot
- mkdir /mnt/bt5 (needs to be done only first time to create the folder)
- qchroot /home/user/Mydocs/bt/bt5.img /mnt/bt5/ (when you see root@chroot you are good to go)
- export USER=root
- vncpasswd (Needs to be done once for setting up a password)
- vncserver -geometry 800x470
- startvnc
Now go back to open VNC and leave terminal open in background
In VNC use this settings :
The adress is 127.0.0.1:590x
x Is the number you see when you typed "startvnc" inside BT5. You will see something like this :
"Log file is /home/user/.vnc/Nokia-N900:x.log"
Mine was "Log file is /home/user/.vnc/Nokia-N900:1.log", so i have 127.0.0.1:5901
Enter password you choosed before, log in have fun.
When done and VNC is closed, do this to shutdown BT5:
Go back to opend terminal
- stopvnc
- exit
- qumount /mnt/bt5
- exit
- exit

Hope i did not miss something, will put the link with the img as soon as it is up (bout 1h30 left)
Will be away until tonight, hope it works.