Script for grabbing images and make them responsive, change dimensions accordingly.

images.js

const IMAGES = document.querySelectorAll("img");
const SIZES = {
    showcase: "100vw",
    reason: "(max-width: 799px) 100vw, 372px",
    feature: "(max-width: 799px) 100vw, 558px",
    story: "(max-width: 799px) 100vw, 670px"
};

function makeSrcset(imgSrc) {
    let markup = [];
    let width = 400;

    for (let i = 0; i < 5; i++) {
        markup[i] = imgSrc + "-" + width + ".jpg " + width + "w";
        width += 400;
    }
    return markup.join();
}

for (let i = 0; i < IMAGES.length; i++) {
    let imgSrc = IMAGES[i].getAttribute("src");
    imgSrc = imgSrc.slice(0, -8);
    let srcset = makeSrcset(imgSrc);
    IMAGES[i].setAttribute("srcset", srcset)
    
    let type = IMAGES[i].getAttribute("data-type");
    let sizes = SIZES[type];
    IMAGES[i].setAttribute("sizes", sizes);
    console.log(type);
}