index image grid – responsiveness – in York Template

Site URL: http://bobbyzdesign.com

Im working in the York template but can't seem to find setting to have grid images be responsive like this example:

https://www.winstonhacking.com

I've seen on a few other portfolio sites – Is this a custom code? or should I just reset style to default settings? 

I currently have my index images set to  "cropped" 

Any help would be much appreciated!

The index images for the second site  are mostly .gifs. It takes them a certain amount of time to load. Just what response. effect do you want? 

Try adding to Home > Design > Custom CSS

@media screen and (max-width:640px) {
article.index-item.has-item-image img {
    width: 100% !important;
    height: auto !important;
    left: 0 !important;
    top: 0 !important;
}
}