Thanks Ismael, but this is not what I had in mind. The provided code changes the thumbnail image within the post article.
I’d like to change the thumbnail image on this page https://www.annapinnock.co.uk/articles/ which is a page that list all the articles. At the moment it’s still 81x81px. Could you please help? I need to remove the code you provided as it messes up my blog article.

Many thanks,

Anna