There is not a facility, in Dreamweaver to check images with no link to them.
However you could set up a 'dummy' site. Copy all the pages to it (Don't update links), but NOT copy the images.
Then go to Site / Check links sitewide (ctrl +F8). This will find all the 'missing' images.
Therefore if an image is NOT on this list, it is NOT required.
Hope this makes sense.