Blog post Featured Image displaying on blog post page

Hello, I need to use Featured Image for each blog post so that the image displays on blog post grids on pages but when I click into a single post the Featured Image is also displaying at the top of each blog post page, and not in a nice way.
I would like to control this and have searched forum to find others with similar goal. They were provided a css snippet to add to Highend > Highend Options > General Settings > Custom CSS, such as

.single .post-featured-image { display:none !important; }

or

.page-id-739 .featured-image {
display: none;
}

But neither of these options work for me. (no clue if the page-id-739 was appropriate for my specific site or not, no idea how to identify the page id for my blog post pages)
Is there something else in play that could be controlling this? Other code I could try?
Many thanks!!