How to expand the image size with keeping ratio.

How to expand the image size with keeping ratio.

I would like to keep the image ratio when expanding. I would like to set height 10% of vertical screen size and adjust width according to the percents of expanding.

for example

screen size is 480 * 600
original image is 20 * 40

How can I make it ?

"#Button":{
       image:"/img/text.jpg",
    left:0,
    height:'10%',width:'auto'
    backgroundColor:"red"
},