Convert your image src https://cdn.shopify.com/s/files/1/0234/8017/2591/products/young-man-in-bright-fashion_925x_f7029e2b-80f0-4a40-a87b-834b9a283c39.jpg into Base64 ULR format and than convert Base64 URL into javaScript File Object.

***Here is the code for converting "image source" (url) to "Base64".***

let url = 'https://cdn.shopify.com/s/files/1/0234/8017/2591/products/young-man-in-bright-fashion_925x_f7029e2b-80f0-4a40-a87b-834b9a283c39.jpg'
const toDataURL = url => fetch(url)
      .then(response => response.blob())
      .then(blob => new Promise((resolve, reject) => {
      const reader = new FileReader()
      reader.onloadend = () => resolve(reader.result)
      reader.onerror = reject
      reader.readAsDataURL(blob)
     }))


***Here is code for converting "Base64" to javascript "File Object".***

  function dataURLtoFile(dataurl, filename) {
     var arr = dataurl.split(','), mime = arr[0].match(/:(.*?);/)[1],
     bstr = atob(arr[1]), n = bstr.length, u8arr = new Uint8Array(n);
     while(n--){
     u8arr[n] = bstr.charCodeAt(n);
     }
   return new File([u8arr], filename, {type:mime});
  }


*** Calling both function ***

  toDataURL(url)
  .then(dataUrl => {
     console.log('Here is Base64 Url', dataUrl)
     var fileData = dataURLtoFile(dataUrl, "imageName.jpg");
     console.log("Here is JavaScript File Object",fileData)
     fileArr.push(fileData)
   })

The simplest way is to use a canvas element and then invoke a download action allowing the user to select where to save the image.

You mention that the image is large, but not how much - be aware that with canvas you will also run into restrictions when the image source starts to touch around the 8k area in pixel size.

A simplified example (IE will require a polyfill for toBlob()).:

• Load image source via input
• Use File blob directly as image source via URL.createObjectURL()
• When loaded, create a temporary canvas, set canvas size = image size and draw in image
• Use toBlob() (more efficient on memory and performance and require no transcoding to/from Base64) to obtain a Blob.
• We'll convert the Blob to File (a subset object and it will reference the same memory) so we can also give a filename as well as (important!) a binary mime-type.

Since the mime-type for the final step is binary the browser will invoke a Save as dialog.

document.querySelector("input").onchange = function() {
  var img = new Image;
  img.onload = convert;
  img.src = URL.createObjectURL(this.files[0]);
};

function convert() {
  URL.revokeObjectURL(this.src);             // free up memory
  var c = document.createElement("canvas"),  // create a temp. canvas
      ctx = c.getContext("2d");
  c.width = this.width;                      // set size = image, draw
  c.height = this.height;
  ctx.drawImage(this, 0, 0);
  
  // convert to File object, NOTE: we're using binary mime-type for the final Blob/File
  c.toBlob(function(blob) {
    var file = new File([blob], "MyJPEG.jpg", {type: "application/octet-stream"});
    window.location = URL.createObjectURL(file);
  }, "image/jpeg", 0.75);  // mime=JPEG, quality=0.75
}

// NOTE: toBlob() is not supported in IE, use a polyfill for IE.

<label>Select image to convert: <input type=file></label>

Update: If you just are after a string (base-64 encoded) version of the newly created JPEG simply use toDataURL() instead of toBlob():

document.querySelector("input").onchange = function() {
  var img = new Image;
  img.onload = convert;
  img.src = URL.createObjectURL(this.files[0]);
};

function convert() {
  URL.revokeObjectURL(this.src);             // free up memory
  var c = document.createElement("canvas"),  // create a temp. canvas
      ctx = c.getContext("2d");
  c.width = this.width;                      // set size = image, draw
  c.height = this.height;
  ctx.drawImage(this, 0, 0);
  
  // convert to File object, NOTE: we're using binary mime-type for the final Blob/File
  var jpeg = c.toDataURL("image/jpeg", 0.75);  // mime=JPEG, quality=0.75
  console.log(jpeg.length)
}

<label>Select image to convert: <input type=file></label>