When you open your posted question in Houzz just look down a bit and you will see the "Comments" sub head below. Your own icon will appear there next to a box that says "Write a comment" ... where you can type in your response. Then hit "submit". Note that you can delete or edit your comment for a period of time after submitting too. Hope that helps.

But at least on my iPad there is no way to respond to a specific comment. People just add theirs at the bottom of the list, maybe including the name of the person replying to.

Hmmm. I don't have the app yet. I just view in the browser. Under "Your Houzz" just see the sub head "Activity" to zero in on responses or look at "Posts you started" as well to see replies. But I'm a techno Dinosaur, so perhaps there's a different way in the app?

This is a screen shot on my iPhone. (I use the Houzz app.) See the little box up top with the triangular thing coming off the lower corner? That's the comment box, you should be able to click on that to post a comment and from there can attach a photo, too.

Dang it, never mind, the comment box apparently didn't show up on my screen shot! Gah! Sorry!