Locate and select any '.dmg' file.
Do a 'Get Info' ['Command I', or 'Finder' - 'File, Get Info'].
Once the 'Get Info' window appears - locate the 'Open with:' section.
From the 'Open with:' popup menu - select 'DiskImageMounter', then click on the 'Change All' button, and finally - click on the 'Are you sure you want to change ...' Dialog Box'es 'Continue' button.
If 'DiskImageMounter' is not listed (in the popup menu), select the 'Other...' menu item.
Go to '/System/Library/CoreServices/'. Here you should fine 'DiskImageMounter'. Select it, then the 'Change All' button, and finally, the 'Continue' button.