This post has no cover image, on purpose — compare its card on the blog index with the one that has a cover. 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, however large the original file is.
Text continues after the image at the same reading width, so the rhythm of the page is unbroken.
A wide image
The next one is marked wide, so it breaks out of the text column and spans the full content width. Useful for a map, a manuscript page, or a panorama.
And the prose resumes at the reading measure, exactly as before.
An image without a caption is left as a plain image, not a figure:
That is the whole syntax: add a quoted caption to get a figure, and start the
caption with [wide] to make it span the full width.