This post has no cover image, on purpose. It does contain images in the body, to check how they sit in prose.
An image at the reading width
Below is an ordinary image with a caption. It stays inside the text column and never grows wider than the measure.
Text continues after the image at the same reading width.
A wide image
The next one is set to Wide width in the editor, so it breaks out of the text column and spans the full content width.
And the prose resumes at the reading measure, exactly as before.