It works!
We have built a new partition structure with 2 PRIMARY partitions: sda1 for the OS with boot flag on, and sda2 as Swap.
We have restored on sda1 the image we had and it works! (we had to make a few changes on the saved image before restoring it and on fstab later on, but this is another story)
That's all!
Thanks for your help, your very fast and successful help!