[Turtle91]

Thumbnails are generally handled by the reading device/app so you’d need to follow whatever requirements they impose.

Assuming the original epub worked properly and all you did was swap the image, then I would make sure the image references were exactly the same. Something as simple as a capital letter in the file name can cause an error as some software is more strict than others.

"Cover.jpg" does not equal "cover.jpg"

Also, some software uses the first image in the file list as the thumb. Capital letters can cause the image order to be rearranged.

Logo_publisher.jpg could appear before cover.jpg

For simplicity and consistency, I make sure to use all lower case letters in image file names and use underscores instead of spaces. I also make sure the cover image comes first alphabetically. In Sigil that is shown by the cover image appearing at the top of the list in the book browser pane.