Allow for a single static image for all blog posts that don't have their own image

[choldgraf]

Describe the feature

Currently, I believe there are two image options for ablog:

• No image: no blog post image
• blog_auto_image: which will display the first image only if it exists

It would be useful if there were a default image fallback option, in case a person wanted images, but didn't have them in their blog post.

Ideas

I can think of three ways you could do this:

1. Allow linking a static image that would serve as the image unless over-written by the post directive, or the auto image
2. Auto-link the site logo following a similar logic
3. Check whether there is og:image set in the page's metadata (if building on HTML) and set that as the blog post image if it exists.