1. watermark-designs.imgix.net and watermarkdesign.com are 2 different domains.
Are you sure you have posted / used the correct URL? 

2. Can you provide a working URL from a single image?
3. But basically you get a 403. Assuming
<https://watermark-designs.imgix.net/products/images/> would be the URL you
intend to use, then you can't. Unless you are properly authenticated to this
website. If you would have login credentials, then you could go the next step.