LinkedIn and Facebook work in a similar fashion when thumbnails are considered. They look for OpenGraph tags that contain images and may process them properly to show thumbnails. However, sometimes LinkedIn doesn't see the image the first time it looks over your link. When LinkedIn fails to recognize the thumbnail, the post will cache in LinkedIn's system for a week. This means that any time we post that link, it will post exactly the same as it did the first post.
How to fix this?
LinkedIn, unfortunately, provides no guarantees as to how to process images to their systems. However, the best chance at getting a proper image from that service is to change the link using some type of append. Essentially, you need to post a set of characters after the link that will cause LinkedIn to recognize the link as brand new. Strikingly's article provides a sample set of characters to use.