This happens because sometimes LinkedIn servers don’t load the thumbnail images in time, and display nothing (black) instead.
Please note that just because you see this blank thumbnail in your LinkedIn inbox, doesn’t mean that others will see this. Oftentimes, the thumbnail image does indeed load and the end user sees it.
To try to re-process the thumbnail image in your inbox, try reloading the page. To reload the page you can click the reload icon on your browser or use a shortcut (Mac: CMD R, PC: F5)