You can make disk dumps from individual partitions or from a hard drive that contains more than one partition. The ladder has the advantage that you can restore the computer by dumping back the image onto the same or another hard drive. But if you want to access an individual partition from a hard drive image, you need to know where the partition boundaries are. You can note them down from the output of fdisk. However, fdisk -l gives the partition boundaries, but the unit is sectors. You need to multiply this number by the logical sector size to get the partition boundaries in bytes.
Alternatively you can use parted (after invoking parted, execute commands unit, B, print in that order) to display the partition boundaries in byte. parted can be also used on a disk dump, e.g. parted /home/backups/diskdump.img to determine the partition boundaries.
jeltsch@Michael:~$ sudo parted /home/backup/diskdump.img
GNU Parted 2.3
Welcome to GNU Parted! Type 'help' to view a list of commands.
Unit? [compact]? B
Disk /home/backup/diskdump.img: 160041885696B
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 1048576B 524287999B 523239424B primary ntfs boot
2 524288000B 80282124287B 79757836288B primary ntfs
3 80283171840B 160041009151B 79757837312B extended
5 80283172864B 122626684415B 42343511552B logical ext4
7 122626768896B 157924769279B 35298000384B logical ext4
6 157932322816B 160041009151B 2108686336B logical linux-swap(v1)
Then you just mount the image as a loop device with the additional parameter offset using the start boundary of the partition you want to mount:
sudo mount -o loop,ro,offset=524288000 /home/backup/diskdump.img /mnt