Well this is exciting! Here are a few shots of Thorin leading an army through a gateway. Is that Dain next to him (or possibly Thrain)? Given the complete lack of grey in Thorin’s hair, and the fact he’s holding his pre-Orcrist sword, I’m assuming this is a flashback. But to when, I really don’t know. It would be easy to guess it’s from the battle of Azanulbizar, if you limit it to only what we know is in canon, but since they may not follow canon, it’s possible this is the gate of Erebor, and the coming of Smaug (which would mean Thorin was quite a bit older during this event than the official timeline).
What does everyone think?
Here is an additional image of what is certainly a king of dwarves (but which one?) seated on a throne.