I haven’t used that API but I’m guessing it might need the photo block, not the url. You may need to check the documentation for that.
Have you tested this with a mobile device?
@codeswept might have a better idea since they have worked on an OCR app
And just an aside since I changed the size of the image you posted to make it a bit smaller, you can do that with any image you paste/upload in the forums by adding a comma followed by a percentage after the resolution. For example, I added “,40%” to yours:
See this for more info.