here's how u do it:
1. u gotta have the pic on ur computer, so download it.
2. when replying, notice 'file attachments' below the post
3. browse ur computer for the pic and click on 'add this attachment'
4. click on 'add into post' and add reply. if u dont want it, just click 'remove'
Its the simplest way, or as Jace said, just attach it like this