The error Failed. Response code = 500. Response message = Internal Server Error occurs when Shopify is NOT accepting the image upload.
The error is caused by Shopify. You may check this by going to Shopify Admin Page:
Product > Choose A Product > Media > Add Media From URL >
input: Image URL
We are not sure why some images are ok. We can do is set up Image CDN to load the images.
We are not sure why some images are ok. We recommend to setup Image CDN to load the images. You can refer here for the setup.