It can be seasonal for the winter, videogame-themed for the Nintendo part or nature-themed for the Valley part.
I don't wanna impose a technique or style, just suit yourself. You might look around the forums to get some ideas though.
As for size, I'm thinking: 500-900 pixels wide and 100-300 high.
If you believe it'd be better to have it rather small or rather big, don't hesitate to say so. See, I'm not just asking for the banner, I'm asking for the kind of banner you want as well.
Okay, so here I'll post the banners we have so far. This post should be updated whenever we get a new one.
>Gaming-themed by malesretmit12, NK64
>Winter-themed by Feld0, YAF (uncropped original version 0_o )
Thanks to both of you for your time and efforts. Especially Feld0 for being such a perfectionist, feedback freak. XD