Please read on the mount syntax. Particularly this bit:

(ii) When mounting a file system mentioned in fstab, it suffices to give only the device, or only the mount point.

Your image is not in fstab so you need to specify all operands. Try something like this as root for size:

mount -t ext3 /home/user/MyDocs/debian-m5-estel.img.ext3 /.debian

(Assuming /.debian exists and is where you want the image mounted. You can use any directory but I recommend using an empty directory to avoid issues.)