Google may override the business owner selection of the Cover photro, typically the process is automated and the alternative cover photo selected by Google may not be the most appropriate like in this case.
Something to keep in mind is the quality of the image. Your selection for cover photo is a low quality image, only 1000 x 556 pixels with noticeable jpeg compression artifacts.
Upload a higher quality picture to see if your new selection can replace the one selected by Google from your web site.
Try to crop it to make it more square, instead of rectangular. Avoid the tail light trails.
treebles, Google My Business Top Contributor