In the forum, you click the icon that says "quote post" - it's the second of the four buttons right below the words in the post. Hope that helps!
In the gallery or in the forums? Here in the forums, at the bottom of the person's post, there's a "quote post" button. Click that, and it should bring up a reply box with their post in it. You can then type below it.
In the gallery, I don't believe there's a way to respond directly. You can click on that person's username (under their photo), then click on "forum profile" and then "send a private message."
In the gallery you can return the love by commenting on one of that person's layouts! Clicking on the username will take you to that person's profile. If you click on the number where it says "number of photos," it will take you to that person's gallery and you can choose a layout to comment on.
