How did they get this screenshot?

How did this screenshot occur? It's either Solaire or another player dressed as Solaire. Either way, what are the circumstances that let a phantom exist at the Sunlight Altar- I thought it would be blocked with fog gates...

I have been summond at the parish and ended up figting on the dragon bridge against red phantoms. So its not that hard.

It was a promotional screenshot, created before the game's release. It was likely either planned for him to be summonable there at some point, or they just customized the code a bit for a theatrical shot similar to the ones that showed Anastasia sitting outside of her cage at Firelink Shrine. Either way, it is not currently possible to do in the game.

I've been summoned at the altar a number of times. When cooping with someone specific I often put my sign around back of the altar on the left side, so random people are less likely to find me. Don't think I've ever seen fog gates there; there's one that appears in the doorway leading to the ladder down to the burg bonfire, and maybe another one at the far end of the bridge somewhere.

I agree that this looks like a promo because a regular phantom won't be that detailed even if dressed as solaire, but you should be able to do it just like that if you want.

actually you can see armor details on phantoms with proper lighting and binoculars, its just a nice screen shot.