I have resolved the problem. It was rooted in the permissions arrangement. Transferring data from the old Mac Pro to the new iMac created another user and my photo files were erratically authorized in terms of which users had read or read/write access. I tackled some pictures individually but that didn't seem to completely resolve the issue. Then I noticed an option in the Sharing and Permissions section of the Info window. It appears when you click on the Gear icon that is next to the add or delete buttons. I clicked on my main identity, changed to read/write and then clicked on the Gear Icon which gave me the option to "Apply to enclosed items". That switched all the items so that both identities now have read/write access.
Hope this helps others who may encounter similar problems.