Does Font Smoothing smooth images too?

What you're seeing in Preview is unrelated to Font Smoothing.

When you open an image at anything but the "native" resolution--that is, 1 pixel onscreen = one pixel of the image--the computer needs to do SOMETHING to adjust it.

Most apps do a form of interpolation to either make images being displayed at larger than native resolutions blurrier but smoother, or ones that are being shrunk look smoother.

Preview shows you a completely un-antialiased version for a moment before it does its routine. It can look worse (if the image is being scaled up) or sharper (if shrunk), but at most sizes it will be crisper but "grainer." Photoshop always does the grainy version, to preserve colors, but to most end users with most images the smoother version looks better.

I don't think you can turn this off, but you can just display the image at the native size (View --> Actual Size or Command-0), which will look as crisp as possible. Alternately, find a different image viewer whose scaling routine you prefer.