Image won't validate



I am trying to integrate Twitter Cards into my Joomla site. The site is I am using K2 for article publishing.

I have used various extensions as well as inserting the meta tags into the article directly to no avail. I’ve tried running the validator tool to check what’s going wrong. I keep getting an error that the image URL is invalid.

The article page I’m testing is:
The image to be used for the card is:

Is there any reason why this image won’t work with Twitter cards? Any help would be appreciated.

Many thanks,
Rich Brown