Disk image files on the Mac OS X are DMG files. They are like virtual images of disks that come with software that is downloaded from the Internet. They contain a copy of the application you downloaded and after they mount on your Desktop, you can remove the application inside by dragging and dropping it in your Applications folder. However, once you remove the application the disk image remains. You have to manually delete the disk image from your Mac with a few simple steps.
Drag any files out of the disk image and drop them into your Applications folder.
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Close the program that was inside of the disk image if you had it open.
Highlight the disk image folder by clicking it with your mouse once.
Drag the disk image folder to the Trash in the lower right hand corner of your Dock. The Trash icon will no longer be a trash can, it will be the Eject Disk icon found on your Mac's keyboard, a triangle with a line underneath it.
Drop the disk image file on the Eject Disk icon. The disk image is now deleted from your Mac.