Use an SVG for the stack image.

Image by @mdo.

site/docs/4.1/assets/img/bootstrap-stack.svg

+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="1024" height="860" viewBox="0 0 1024 860"><defs><linearGradient id="c" x1="50%" x2="50%" y1="0%" y2="100%"><stop offset="0%" stop-color="#5C24AE"/><stop offset="100%" stop-color="#30135A"/></linearGradient><path id="b" d="M355.967 242.807l-322 216.395c-44.275 29.754-44.275 78.443 0 108.197l322 216.395c44.275 29.754 116.725 29.754 161 0l322-216.395c44.275-29.754 44.275-78.443 0-108.197l-322-216.395c-44.275-29.754-116.725-29.754-161 0z"/><filter id="a" width="108%" height="112%" x="-4%" y="-4.3%" filterUnits="objectBoundingBox"><feOffset dy="10" in="SourceAlpha" result="shadowOffsetOuter1"/><feGaussianBlur in="shadowOffsetOuter1" result="shadowBlurOuter1" stdDeviation="10"/><feColorMatrix in="shadowBlurOuter1" values="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.25 0"/></filter><linearGradient id="f" x1="50%" x2="50%" y1="-17.303%" y2="100%"><stop offset="0%" stop-color="#7331D4"/><stop offset="100%" stop-color="#461B84"/></linearGradient><path id="e" d="M355.967 132.807l-322 216.395c-44.275 29.754-44.275 78.443 0 108.197l322 216.395c44.275 29.754 116.725 29.754 161 0l322-216.395c44.275-29.754 44.275-78.443 0-108.197l-322-216.395c-44.275-29.754-116.725-29.754-161 0z"/><filter id="d" width="108%" height="112%" x="-4%" y="-4.3%" filterUnits="objectBoundingBox"><feOffset dy="10" in="SourceAlpha" result="shadowOffsetOuter1"/><feGaussianBlur in="shadowOffsetOuter1" result="shadowBlurOuter1" stdDeviation="10"/><feColorMatrix in="shadowBlurOuter1" values="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.25 0"/></filter><linearGradient id="i" x1="50%" x2="50%" y1="0%" y2="100%"><stop offset="0%" stop-color="#905BDD"/><stop offset="100%" stop-color="#5521A0"/></linearGradient><path id="h" d="M355.967 22.807l-322 216.395c-44.275 29.754-44.275 78.443 0 108.197l322 216.395c44.275 29.754 116.725 29.754 161 0l322-216.395c44.275-29.754 44.275-78.443 0-108.197l-322-216.395c-44.275-29.754-116.725-29.754-161 0z"/><filter id="g" width="108%" height="112%" x="-4%" y="-4.3%" filterUnits="objectBoundingBox"><feOffset dy="10" in="SourceAlpha" result="shadowOffsetOuter1"/><feGaussianBlur in="shadowOffsetOuter1" result="shadowBlurOuter1" stdDeviation="10"/><feColorMatrix in="shadowBlurOuter1" values="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.25 0"/></filter></defs><g fill="none"><g transform="translate(75 23)"><use fill="#000" filter="url(#a)" xlink:href="#b"/><use fill="url(#c)" xlink:href="#b"/></g><g transform="translate(75 23)"><use fill="#000" filter="url(#d)" xlink:href="#e"/><use fill="url(#f)" xlink:href="#e"/></g><g transform="translate(75 23)"><use fill="#000" filter="url(#g)" xlink:href="#h"/><use fill="url(#i)" xlink:href="#h"/></g><path fill="#FFF" d="M558.273 447.667L308.036 279.5l97.982-65.847c42.83-28.784 96.789-31.483 134.245-6.311 26.044 17.502 31.214 46.615 11.444 65.724l1.389.934c38.324-19.932 84.107-18.527 117.396 3.845 44.046 29.6 38.671 68.419-14.561 104.193l-97.658 65.629zM447.112 331.01l49.942-33.562c36.935-24.822 42.31-48.249 15.224-66.451-24.798-16.665-55.49-14.453-85.851 5.95l-59.641 40.081 80.326 53.982zm176.532 35.663c37.63-25.289 42.136-48.832 13.203-68.276-28.932-19.444-64.163-15.614-104.042 11.186l-58.789 39.508 87.92 59.084 61.708-41.502z"/></g></svg>
\ No newline at end of file

site/index.html

@@ -6,9 +6,7 @@ layout: home
   <div class="container">
     <div class="row align-items-center">
       <div class="col-6 mx-auto col-md-6 order-md-2">
-        <img class="img-fluid mb-3 mb-md-0" srcset="{{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-stack.png,
-                                                    {{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-stack@2x.png 2x"
-                                            src="{{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-stack.png" alt="" width="512" height="430">
+        <img class="img-fluid mb-3 mb-md-0" src="{{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-stack.svg" alt="" width="512" height="430">
       </div>
       <div class="col-md-6 order-md-1 text-center text-md-left pr-md-5">
         <h1 class="mb-3 bd-text-purple-bright">Bootstrap</h1>