Images do not get scaled down; they're embedded as they are (which means large images take a long time to download), but their maximum displayed width might be smaller. Since the introduction of responsive design, this depends on the browser width.
The line above is a 3200 x 10 image which (in my browsers) is resized to 677 pixels wide. There's a
max-width is hardcoded to 1100 pixels with 24 pixels padding (left and right), so there is 1052 pixels left; the
div#mainbar has a width of
calc(100% - 300px - 24px); which amounts to 728 pixels. The post body shares that space with the vote cell (36 pixels wide, as specified in the vote button SVGs), with a padding of 15 pixels between them. That leaves 677 pixels for the post body (and the image).
At a screen width of 816 pixels and lower, the horizontal padding of the
div#content changes to two times 16 pixels, and the
width of the mainbar changes to 100%, so then the width is (screen width - 32 - 36 - 15), which has a maximum of 733 pixels. So interestingly the image will be displayed a bit wider then, at least just below a screen width of 816 pixels.
That's here on Meta.SE; there are some sites with a border in their design, even on the Q&A page, e.g. Photography. Those borders eat away one or two pixels. I'm not sure if your interested in that order of variations; if you are, you browser's developer tools are your best friend. Science Fiction & Fantasy's vote buttons have some extra margin (2px left and right), further reducing the image space.