Backup your database: DB Backup/Restore
Open the product file for which you want to import new detailed images (do not include products for which you already have detailed images because they will be duplicated)
Save this file under new name. Delete all the columns from it but 2:
Save it as text file
Go to "Import images"
Set "Column order" to:
Do not check "Get columns names from the first row of CSV file"
Select "CSV delimiter"
Browse for your import file
Click "Import images"
If you did not get any error messages, check if images were imported properly by going to the store and checking some products
If something wrong - go to "DB Backup/Restore" -> restore your database and send the file you tried to import to us or upload it via "Files" interface for us to check.