According to official LinkedIn guidelines, publishers should use a 1.91:1 ratio and 1200x627 px size when uploading photos to a LinkedIn page. The image must be at least 200 pixels wide. If your image to too small or large, you can use this free web tool to resize your image to the correct size for the LinkedIn feed. Just upload the image to this template, and Kapwing will crop or fit it to the right LinkedIn image size.
Also works for LinkedIn video thumbnails.
Spec: Image downloads as a JPG, 1200x627 pixels.