The Odessa theme displays the featured image (which is the first image in your post) above the post header on single post pages. You can see an example here.
If you want to remove it, go to Theme > Customize > Advanced > click on "Links" to expand the dropdown > scroll down to the bottom and select "Add CSS" > add this to the "Add custom CSS" field:
.single-entry-thumbnail {
display: none !important;
}