If you have All in One SEO on your blog, makes sure you fill out the bottom parts of the post. Scroll down from your edit screen. You'll see "All in One SEO Pack." Under that are two tabs: Main Settings and Social Settings. Under Social Settings, scroll down a little bit and you will see a place to choose an image. Click one of the images. Make sure you save draft or update your page if already published. Facebook is a little strange. You may have to debug your post URL. Here's the Facebook Debugger URL. Put in your post URL and hit debug. You may have to do it more than once. Good Luck!