2D art requirements for game release

Hi everyone,

I started developing my 2d tower defense game a while ago, when i was almost done with it i hired a freelance artist to do all the art needed for the game to be released. Now i am not experienced with the art requirements for such project, but i wanted the art to be good for deployment in landscape view for android and iOS. So what aspect ratio / resolution should the art be in? and what should i ask the artist for to get a quality assets for game release.

Thanks,

 

images should ideally be powers of 2 ... 16,32,64,128, 256, 512,1024,2048 .. etc

i dont really do 2d or mobile but thats a little i can help, suppose the artist would know so anyway
and i dont think its a Huge deal (it could be though if your design was complex), but its like optimal