Save public photos at the size they were uploaded, not a preview.
A screenshot gives you whatever your screen holds. Pasting the link gives you the picture itself — the same JPEG the author uploaded, at its full pixel size.
Up to the maximum the post contains, without the compression a screenshot adds.
Carousels return each image separately, so you can save one or all of them.
Nothing to log into and nothing to install. Paste and save.