How to Reimage a Macintosh Computer
Mac OS X computers come with an application called Disk Utility that can be used to create disk images from existing drives. A disk image is a file that, when opened, appears on the Desktop as if it were another hard drive; it is commonly used to distribute software over the Internet. Disk Utility can make a copy of your hard drive as an image for later reference.
Attach an external hard drive to your Macintosh that has at least as much storage capacity as the space used on the drive you are imaging. You can skip this step if you already have a second drive with enough free space.
Launch the Disk Utility application from the Applications/Utilities folder.
Select the drive you wish to image by name in the list on the left-hand side of the Disk Utility window. This list will also include drives by their serial number. You should usually ignore these in favor of the named drives; however, select the drive by serial number if you want to create a single image of a drive with two or more partitions.
Choose "File" > "New" > "New Disk Image from...", which will display the name of the drive you have selected.
Select the external drive in the Save dialog. If you wish, select a compression format and encryption method. Compression will create a much smaller disk image which still contains all the information on your original drive. Encryption will require a password before the disk image can be opened; if you lose this password, you will not be able to access the contents of the disk image.
Click "Save" to create the disk image. Disk imaging can take a long time, especially if you are creating a compressed or encrypted disk image. You can continue to use your Macintosh during this time, but it will run more slowly due to the disk access required by the imaging.