losetup can be used to associate a file with a loopback device. This way, you can make files and use them to experiment with LVM, mdadm, or whatever else might require block devices.
List loopback devices that are currently associated with files
losetup -a
List the next available loopback device
losetup -f
Associate a loopback device with a file
losetup /dev/loop0 /path/to/file.img
Disassociate the loopback device from the file
losetup -d /dev/loop0
You can also mount just a partition within an image file
lomount -diskimage /path/to/your/backup.img -partition 1 /mnt/foo
when refering to the file for mounting or formatting, etc., use /dev/loop0 to refer to /path/to/file.img