One of the interesting things that can be done with hdiutil is attaching a read-only image using a shadow file.  Here's how:

 First, attach the read-only image as with the shadow option:

hdiutil attach -owners on your.dmg -shadow

After typing this command, the image will be attached as a new device to your operating system.  Usually this is shown from the command line.

Next, copy over any items, or modify anything on the read-only image.  Detach the image when finished:

hdiutil detach /dev/disk#

Almost done!  Now, convert the image back to read-only

 hdiutil convert -format UDZO -o new.dmg your.dmg -shadow

 Guess, what you now have a brand new dmg, and the time and space required to convert the image to read-write has been eliminated.