A "Disk Image" is a copy of the drive saved in file. Disk Image is used for backup/restore purposes. Using Active@ Partition Recovery you can create "Disk Image" for the whole Hard Disk Drive or for a particular logical drive.
Another way (available in Professional version only) is to run software with the parameter -image. For example:
Resulting Disk Image is saved in the following files: Drive_80h.hdd, Drive_80h.001, Drive_80h.002...
Each file (except the last one) has size 1GB. MS-DOS does not support files more than 2GB in size. Due to this reason the files are separated in 1GB file size. If you want the Disk Image in one file - merge them later on manually.