I don't know, he can't know for certain we'll win.
This specific wolf looks a bit dragonish and fiercer and since dragons & gods go together I assumed it's Solas.
The dragon looks like a dragon's foot to me, it may indeed not mean killing Flemeth but perhaps some other thing? Plus, what made us decide that he'll approach Flemeth only if the orb is broken?
Also, I sort of refuse to buy it's that simple XD, since this appears post-credits I figure it is a way to give us hint us of what happened, like the conversation with Cole.
It's incomplete and therefore seems like BW wants us to fill the blanks (they're evil like that).
I thought they might have done it to mess with us.

That is an excellent shot of it btw. Thanks for posting it.
I'm of mind he returned to Skyhold somehow after the fight and after the Flemythal thingy. Yes I know Leliana's report suggests that he was not to be found after the defeat, and everything seems to indicate that he did not come back with you. But that doesn't mean he didn't come back. Don't forget there's an Eluvian in Skyhold, and there was an activated Eluvian right there in the post credit scene. I think it's possible he used it to return quickly and unnoticed, and that's when he began this painting. Perhaps as a clue/warning or even a subtle cry for help, like he's wanting you to discover the truth so you can do something about it and stop him/save him, But ultimately, decided not to linger and finish it. The thing that makes me think he came back the most, however, is Cole. Outside of his own memory, he typically hears people's hurts in real time, no? And also they usually aren't too far away when he does it.
The only exception I can think of is the CS with the amulet. But in that one, he was focused on the amulet (nearby) to discover the pain that was interfering.
Though how heartbreaking would that be, that in the moment you're listening to Cole, Solas is walking out the gates (or possibly through the Eluvian). So close, but hopelessly out of reach.
Anyway, seeing it so clear now, I believe it is a wolf with a sword'd dragon also. I also think the relative size of the wolf to everything else is very important, but I'm not really sure why.