css - background-size contain AND length -

i have div has background image size set contain. however, image double resolution image retina computers (macbook pro, etc), i'd able make page aware somehow though i'm saying background-size:contain 200x200 image, should treating if 100x100 image. there way this? want this:

{     background-position:center bottom;     background-size:contain     background-size-again:100px 100px; /*???*/ } 

okay should use

background-size:cover  

this force image stay @ high resolution , set element size size want it, example 100px x 100px