Well, it depends.
If your new items belong to the same topic (that is, you have completed more Frostgrave terrain pieces), then it would probably be best to collect them all in the same thread by posting a reply. Makes it easier for readers to follow your update.
If you do something very different, such as starting building a game table or showing your figures in particular, then it would be perfectly fine to open a new thread for the new, different topic.
You can edit your original post title to show that you have added new stuff to the topic, if you want to.