If I'm playing something very structured, like bach, I'll be honest, my mind wanders, I think about school, what i'm doin durin the day, stuff I'd normally think about when I'm not playing piano...
... For other stuff, it's not the music making me see things, usually it's me forcing myself to see what the music portrays. I mean like imagining nighttime while playing a nocturne... If I focus on images that relate directly to the music, I play it better. So like for danzas argentinas, I'd imagine someone, insane, running around. Another example would be jeux d'eau, I imagine standing in a large pool of water only like 2 inches deep, and drops of water cause ripples. Pretty straightforward, what I see usually reflects the title or typical mood of the piece.