If you would like to target only medium screen sizes you can do it in the following way. give width: 120% to your img element and it will overflow and take the full size at the left.
working snippet
.img {
width: 120%;
}
.col-md-6 {
background-color: green;
}
NOTE:
If you want to target all screen sizes media then just use col-lg-* col-xs-* col-sm-* i.e different responsive bootstrap classes.
Hope this helps!