Stumped

A long time member, I rarely use the classifieds - just enough to kind of remember how to do things.
When I post a listing of parts for sale, I'll get an inquiry for a better picture - which not unusual. When I try to reply with a picture, my WORDS can be returned in the reply, but I am unable to send the requested picture. There is no "Upload Photos" option (such as below) offered.

I tried to decipher the FAQ that distinguished "PM" from "Conversation", but still no luck.

How do I send a follow-up PICTURE to a person responding to my ad who is requesting one?
I know this must be simple, but it eludes me.
Thanks,
Mark Koenig