shopify-buy-button-and-wordpress-visual-product-picker.html

<ul id="products"></ul>
<input id="product_id" name="product_id" type="text" />

shopify-buy-button-and-wordpress-visual-product-picker.js

var client = ShopifyBuy.buildClient({
   apiKey: '',
   domain: 'example.myshopify.com',
   appId: '6'
});

var $products = $('#products');
var $input = $('#product_id')

client.fetchAllProducts().then(function(products) {
  products.forEach(function (product) {
    var li = $('<li>');
    li.text(product.title);
    if (product.selectedVariantImage) {
      var img = $('<img width="50">');
      img.attr('src', product.selectedVariantImage.src)   
    }
    li.prepend(img);
    li.click(function (evt) {
      $input.val(product.id);
    });
    $products.append(li);
  })
});