A disk Image is essentially a "drive inside a file". It is merely the enclosure in which the software was shipped.
What you want to see is the actual Application sitting inside your /Applications folder. If you see the entire disk image inside your Applications folder, you did not manage to install it properly.
The disk image is likely still on your drive, unless you deliberately deleted it. Once you see the Application in your Applications folder, you can generally delete the disk image. If you are REALLY cautious, you could launch the Application once to be sure it finished installing everything it needed out of the Disk Image, then delete the disk image to recover that space.