In the Twenty Seventeen theme, a big thumbnail image (featured image) appears above the tile in single post.
If you want to remove the thumbnail image from posts, the simple method is to hide it using the following simple CSS code:
.single-featured-image-header {
display: none;
}You can create a child theme or you can just add this code in the "Additional CSS" section under Appearance > Customize in the WordPress Dashboard sidebar menu.
If you want to hide the thumbnails from the recent posts in the front page (if you set the front page to "recent posts), you can use the following CSS code:
.home .post-thumbnail {
display: none;
}Of course, it's also possible to edit the theme source file.
If you want to run your own personal blog, this new "Twenty Seventeen" theme will be a good choice. If you want a fast and clean premium theme, I personally recommend Enfold (currently used in this blog), X or Divi from Elegant Themes.