First let me say that all of the screenshots are really nice
For the homepage I'd vote for 'Four Tribes 2'. Of the ones showing all four tribes it's the one I like most. Unfortunately Barbarian and Atlantean buildings are a bit underrepresented. But it looks more pleasant to me than the others.
If the voting is close one could consider changing the screenshot on the homepage on a regular (or random?) basis. Maybe once a week/month. Of course I don't know if that would be easy to implement by a script.