It sometimes happens to me too when I share my blog posts, and sometimes the images are there. It's very random and I cannot account for it. But if you never have any images you might want to check th open graph tags. maybe they are not defined properly, so facebook cannot find the images. You can go to https://developers.facebook.com/tools/debug/ and enter your url there. Then your page will be analyzed and you will see what images can or cannot be found and how it should look like when you share. Also I noticed that when I use a sharing button from my wordpress blog the image shows up in facebook, but when I copy/past the post URL the image doesn't show up. To me this is really totally random. I then usually add my own picture