How to turn off the thumbnail inside a post in Wordpress?

Question

Blogs

There is a blog on Wordpress, now I'm choosing a new theme, because. the previous one is tired/obsolete. But I'm still looking for something similar.
I found the theme, installed it, but the thumbnails were not shown. It turned out that for each post you need to set your thumbnail. I did not know this, because my previous theme itself generated thumbnails from the 1st uploaded image.
I started uploading thumbnails and noticed such a problem (maybe only for me this is a problem) that when you add a thumbnail, it is shown both in the announcement and in the full post, but I don’t need it at all, because. There is a picture at the beginning of each entry. In general, it is necessary that the thumbnail is shown only in the announcement of the record, i.e. on the main, similar records, except for the full record.
How can I disable thumbnails in the full post?
PS I noticed that in many topics (not all) the thumbnail is added to the body of the post.

Answer 1

Special settings area: Appearance - Theme Options

Answer 2

The thumbnail in the body of the post is often added in different ways. Therefore, it is easier to open the editor (Appearance -> Editor) and through the search (Ctrl + F) in each file, look for "thumbnail". Further according to the circumstances: For example, through the browser console (F12 in Crome), view the page code, and calculate the location of the thumbnail in the post and in the template file (for example, by the class parameter).