We already downloading onebox images. The issue here is “Google Photos” not using file extension in the image URLs. So we are unable to fetch it from the post content. Anyway we can achieve it by adding a special case for Google. Added to my list.
Why it’s not working 
. It have valid image URL (https://c1.staticflickr.com/1/18/24130109_68f93566b9_m.jpg) inside onebox html.