Если вы google "js image url to base64" , вы найдете много реализаций.
Пример:
/**
* Convert an image
* to a base64 url
* @param {String} url
* @param {Function} callback
* @param {String} [outputFormat=image/png]
*/
function convertImgToBase64URL(url, callback, outputFormat){
var img = new Image();
img.crossOrigin = 'Anonymous';
img.onload = function(){
var canvas = document.createElement('CANVAS'),
ctx = canvas.getContext('2d'), dataURL;
canvas.height = img.height;
canvas.width = img.width;
ctx.drawImage(img, 0, 0);
dataURL = canvas.toDataURL(outputFormat);
callback(dataURL);
canvas = null;
};
img.src = url;
}
Источник: https://gist.github.com/CawaKharkov/1c477d44fcfdf67aea3f