Hi, Paul!
First of all, it looks like you’re using Wordpress for your site, and you are using 2 plugins - Yoast SEO, and JM Twitter Cards. We don’t officially support either of these ourselves but both work well in my experience. The problem is that you don’t need both of them for Cards to work, and right now they are duplicating the information in your header. To see this, if you view the source of your webpage, inside the <head> section you have two sets of references to twitter:card markup, added once by each of these two plugins. I’d disable one of these, probably JM Twitter Cards since the Yoast plugin offers other features as well.
Now the second issue here. You’re using a Twitter Summary Card, and for that to display an image you need a tag in the same section which either provides twitter:image or og:image and a link to the image. I don’t see that in your page, so either there’s something wrong with your plugin (and you’ll need to check with the author why it isn’t adding the link), or you’re not adding the URL to the image in your configuration correctly.
