To image a computer typically means to apply a prepared image of the hard disk, which is stored elsewhere, onto the computer's hard disk.

This method is often used in manufacturing and in larger corporations, where images of the typical configuration(s) are made and maintained, then applied to new computers in a mostly automated fashion.

To re-image probably is meant to apply the same image once again. This process is the equivalent of a complete wipe-out and re-installation, returning the computer to its original state.