Why would they bother showing Bianca's hat left hanging on the easel from which we saw the picture taken? If Bianca was out and about somewhere, her hat wouldn't have been left home with Latias.
Maybe she didn't feel like wearing her hat that day? Contradictory as far as costumes in Pokemon go, but maybe she really didn't want to wear the hat that day
But I think they kept the hat to actually show that Bianca was out. I don't know if this makes sense to you but it's a sort of sign that the hat was all that remained of Bianca in the house. Eh, I'm really not making sense here, but....yeah, I feel it was meant to point towards Latias.
That, and it possibly created some "suspense" to allow for Misty's question at the end.
Or maybe I'm just not convinced that they'd write a scene where a Pokemon has a romantic-ish kiss with Ash. I'm even surprised everyone else so far seems to assume that. But in all my tenure here, I have never seemed to align with what appears to be the majority PAD opinion, so oh well.
Yes, it seems unlikely, doesn't it? But the way I see it, Bianca and Ash virtually had nothing going on between them - I wouldn't expect her to randomly kiss him like that. On the other hand, Latias, throughout the movie, had a fascination for Ash, be it romantic or not, so she makes more of a candidate for an Ash-kisser in my opinion. She was also a girl when she did kiss him, so she's not exactly a Pokemon =P
Also, anyone notice the way Latias runs, particularly at the end? That's a very large hint if not conclusive proof.