I’m suggesting only the full range of tools that are available underOh, you want text formatting tools for the photo's Description box.
(is that “post” or “message” ....I get so confused) ..."Post" or "message" has been argued about here. I tend towards "post a message" - "post" is the action and "message" the object. But I'm not always consistent about it.
As to "subject", that's a short text field that applies to both a "topic" and to each message in the topic. So for what you wanted to say "subject" is correct, the "topic" would be the list of messages sharing that subject text - including this one of mine.
Of course, this is Groups.io usage (jargon) I'm describing. In general English usage the two are much closer to interchangeable.