LinkedIn scrapes the page to find images, it is totally theme independent.
Do try the following with different posts :
1) Disable all plugins and try sharing again.
2) Enable automatic featured image display in posts and then try, most common issue.
3) Lastly, refer http://stackoverflow.com/questions/33009733/no-image-and-title-when-sharing-my-blog-post-on-linkedin
Also sometimes, the servers that pull the images are busy so try at a later date too. If Facebook and twitter are pulling images fine then there could be issue with LinkedIn itself.