# losetup /dev/loop1 /mnt/cifs/backup/sdb2.img 
# losetup -a
/dev/loop1: [0013]:1715743 (/mnt/cifs/backup/sdb2.img)

# file -s /dev/loop1
/dev/loop1: , 44.1 kHz, Stereo

# /opt/e2fsprogs/sbin/e2fsck /dev/loop1
e2fsck 1.41-WIP (27-Apr-2008)
/opt/e2fsprogs/sbin/e2fsck: Superblock invalid, trying backup blocks...
/opt/e2fsprogs/sbin/e2fsck: Bad magic number in super-block while trying to open /dev/loop1

The superblock could not be read or does not describe a correct ext2
filesystem.  If the device is valid and it really contains an ext2
filesystem (and not swap or ufs or something else), then the superblock
is corrupt, and you might try running e2fsck with an alternate superblock:
    e2fsck -b 8193 <device>

# /opt/e2fsprogs/sbin/e2fsck -b 8193 /dev/loop1
e2fsck 1.41-WIP (27-Apr-2008)
/opt/e2fsprogs/sbin/e2fsck: Device or resource busy while trying to open /dev/loop1
Filesystem mounted or opened exclusively by another program?

# losetup -a
/dev/loop1: [0013]:1715743 (/mnt/cifs/backup/sdb2.img)

# lsof -ln | grep loop
loop1      5065        0  cwd       DIR        8,5       4096          2 /
loop1      5065        0  rtd       DIR        8,5       4096          2 /
loop1      5065        0  txt   unknown                                  /proc/5065/exe