Appearance → Customize → Single Post → Show featured image (top). Off by default — the one Single Post part that is.
Why it is off
A featured image earns its place in a listing, where it helps a reader pick. At the top of the article the reader has already picked, and a large picture between the headline and the first sentence pushes the writing below the fold. Some sites want it anyway — a photo-led magazine certainly does — so it is a switch, set to off.
Width
Featured image width decides how far the picture runs:
| Content width (default) | the same width as the text |
| Wider — 100px past the content each side | a gentle break out of the column |
| Container width | the full page container — 1170px by default |
Every one of these is a ceiling, not a stretch. A 700px image on a Container width setting is printed at 700px, centred, not blown up to 1170 and blurred.
So upload big. For Container width you want an original at least as wide as your content width, and ideally twice that for high-density screens.
The caption
If the featured image has a caption in the media library, it is printed under the picture. That is where a photo credit goes.
Note the caption comes from the attachment, not from the post — set it in Media → the image → Caption, or in the featured image panel when you select it. A caption typed into a Caption block in the content belongs to that block, not to this figure.
Posts only
The top featured image is printed on posts. A page with a featured image set does not get one; use an Image or Cover block in the page content, which gives you alignment and overlay options this figure does not have.
The image is not repeated
Turning this on does not remove the picture from the listing — the listing thumbnail and the top image are the same file used in two places, at two sizes. The full-size file is served here and the large size in the listing.