Personally I prefer it to be Latias 
Logically, I think it is Bianca. Firstly, the hat. The hat was supposed to be on Bianca head. But the hat was left, so you cannot say that it is Latias because she didn't wear hat.
Secondly, you cannot say just because she didn't talk and didn't wear hat, she is Latias. Bianca could do it. She should have been disguised as Latias so she could freely do anything without being herself.
Thirdly, the painting is Bianca's. Latias should has no reason to give it away to Ash, I think.
Lastly, why Bianca painted Ash and Pikachu at the first place? I don't think Bianca did it for Latias, since Bianca had started the painting before Bianca met Latias and Ash.
dunno, lol.
