As you possibly know, I am sending automated Tweets whenever I am writing a new blog post, containing the URL of the post. And since I've changed the blog engine, I was always seeing the Twitter card associated with the URL having the same image, the blog tile. So I made changes to display a separate image for each post, by populating the og:image meta property. It didn't work. The Twitter card now refused to show the image. And it was all because the URL I was using was relative to the blog site.
First of all, in order to test how a URL will look in Twitter, use the Twitter Card validator. Second, the Open Graph doesn't specify that the URL in og:image should be absolute, but in practice both Twitter and Facebook expect it to be.
So whenever populating Open Graph meta tags with URLs, make sure they are absolute, not relative.