Safari - Image Scaling

I assume by image scaling you mean zooming in on a particular image.

While I'm sure there are a number of plug-ins available, my all-around plugin of choice for this is SafariStand. If you enable it in the preferences, you can scale the image by press + or -

If you mean "Scale to Fit," I can't remember if that's a contextual menu in Safari by default, but it's definitely there with SafariStand. (Although I'm not finding how to set that to automatically be done.) SafariStand also adds some other cool image things (Show Exif Data), too, so I recommend it.