I have installed Viber and I also see that Viber doesn't show an image for this page.
When looking at the page I see that all the basic OpenGraph tags are correctly configured, also the og:image
I noticed two things:
1. you have not set the og:image size tags (you can configure these in the ochOpenGraph settings (tab OpenGraph)
2. when scrolling down the page source I see a second set of opengrapg tags that will overrule the ones set by ochOpenGraph. These come from your template or from another plugin?
Because it is Viber that fetches the image from the page (we only share the URL with them), I have done some research on how Viber fetches the image: there is no consensus.
Experiences of people are: random image, 1st image in page, the opengraph image, no image.
So just to make sure: can you add the og:image size tags and see what causes the second set of og: tags?