There are two ways people can be added to your "Other" contacts:
You used Google+ and they were in one or more of your Circles
You have emailed them in the past and the setting "Create contacts for auto-complete" is turned on (See: Stop Gmail from automatically creating contacts). Used to be that you couldn't control that setting, so if you've been using ...
The photo could come from Gmail or from https://myaccount.google.com
NOTE: Recently Google announced that both places will use the same photo.
Set profile picture for Gmail and all Google apps
Follow my instruction carefully, you can get the duplicates and merge it successfully.
After you import a new CSV file, Google automatically makes a different label for the imported contacts details.
Delete the label (normally label is named as import and date) keeping all contacts.
After deleting the label successfully , refresh the tab.
Now click on ...
I made an export of Google contacts with .vcf, and imported into mailbox.org.
The photos did migrate.
Google Contacts web interface:
Select all contacts
Drop down menu for all selected contacts
Export as vCard (for iOS Contacts)
Save the file: MyGoogleContactsExport.vcf
Mailbox.org Contacts interface