Second import deletes previously uploaded values for core user picture field

Scenario: I'm importing fields (both profile2 and core) including the core user picture field. Everything works great on the first import. For the second import, I don't include a picture column, but the other columns remain the same. This second import deletes all values imported previously for the user picture field.

I believe the way UIF works is that it will either create new or update - not delete if a field is omitted from the import.