Creating Wallpapers

Hi,

- as an professional artist, I plan to share some of my art work as wallpapers. Free of course, as it's only art work with my signature I put into galleries for sale.

My question/s then is pretty basic:
- I only want to upload the wallpaper (not a theme, but if anyone find something they would like to build a theme around, that's of course no problem), so what size (in pixels) is preferred - is there any resolution preferred - and what file format should I upload in?

Thank ...