Though I myself am not sure about whether or not this leak is in fact real, I see some fairly unconvincing arguments for it being faked.
The first look Sumer shot uses white text where all the others use yellow.
I think you are more than likely wrong about that. These are all photos of a screen, and the difference in text colour in these photos appears to be due to the angle the photo was taken at (screens look brighter from different angles in photos, something that you will notice if you try taking a picture of a screen). An example:
So I'm sceptical of that being evidence against the picture. Just take another look at these, and I will think you will be able to see that the text on all three is in fact likely all the same colour:
The first look Ottomans shot calls them 'Ottomans' with an S on the end. The first look Aztec video did not add the s to the end. That's inconsistent, not an impossible, but an improbable mistake
I will agree this is a funny inconsistency. Possibly evidence for your case, but not necessarily conclusive; this is a very minor thing.
Both the Sumer shot and the ottoman shot use magnitudes brighter backgrounds than any of the other Civs first look screens. Especially the ottoman one, I don't see even a hint of the black and dark grey that dominates, particilarly the edges, of all the other Civs first look screens.
Again, you are forgetting that these are photos taken of screens. Screens will always can look brighter when photographed at certain angles.
Then there's the fact that they built to the ottoman reveal using two 'confirmed Civs' to test the reception, while we can be pretty sure without this screenshot that the Ottoman Empire have not made the cut this time.
Why would you say that? Ottomans seem to me, and several other posters to be the most likely contender for C5 (e.g. AriochIV I remember felt they were most likely, citing that the Ottoman tweet would not have had no reason). On top of the tweet, the picture even looks to a lot of people like it is wearing a turban, meaning it is most likely to be a Muslim Civ like the Ottomans, and not the Mongols. And, as I have laid out reasons for before, the Khmer tweet was not the same. Others just insist C5 is the Mongols because Mongols have been in every other game. What do you suppose C5 is?
I would also remind you, as was pointed out earlier,
this Rome leak quite resembles the
background we have already seen for Rome. And if that one is real, then chances are so are the other two.
Indeed, just a corner with text is easy to fake.
No these would not be hard to fabricate, but why would the person have not come forward yet at this point and posted something along the lines of '

Sorry guys, it was a fake'? What would they be waiting for?