No caption, no boost. Applies retroactively as needed. I apply my own judgement whether an image description in the post counts as a caption (but it usually doesn't imho).
They should move that clarification dialogue to the response to control-V pasting an image in for discoverability. Apart from that minor criticism I'm impressed by the feature set.
@jasonp @feditips (Correction: It *is* in the response to ctrl-V. So *that's* what the "Edit" button does!)