There are several ways to get to the right screen, but one way is to use the Notification Bell icon at the top of the screen. You’ll see notifications there for every activity.
When you see that someone has commented on your art, click on the message in the drop-down window, and it will take you to the conversation.
If you used another method to get to this point, it make look different. What you are looking for is ‘Reply’. Hit reply and type in your thanks. You can also like the comment for DO points. In some windows, you’ll have a ‘Like’ option and can click on that. In other windows, there will be a heart at the top right of the comment. Click on the heart and it will turn red, showing that you have liked it.