Did you ever typed your blog’s homepage address on Facebook? and Facebook showed wrong logo as preview. It happens to me too but if you really want to show right logo of your blog or any specific image on social sites, you can use this meta tag below:
You can just create a unique image just for use on social networks example is this image below:
To add this meta tag in your blog, just add this line below ‘<head>’ tag
<b:if cond='data:blog.url == data:blog.homepageUrl'>
<meta content='YOUR_BLOG’S_IMAGE_URL' itemprop='image'/>
<meta content='YOUR_BLOG’S_IMAGE_URL' property='og:image:src'/>
Their are also other meta tags like Copyright or Keywords but now, they doesn’t matter any more.
Google stopped using keyword meta tags for calculating ranking of blogs.
If you know any other important meta, please mention it on comments below and I will add it ASAP!