Thank you for your reply.
I'll try to explain it better:
Fit to screen: When the image is auto-resized in order to be fully seen in the current screen resolution, regardless if it is in landscape or portrait orientation. In other words, when the image is auto-resized (but not stretched) to fit both width and height of the screen (not the web page).
Fit to width: When the image is auto-resized in order to fit the screen width, but the image height might be larger of the screen height (for example when the image has a portrait orientation), so the user will have to scroll up or down, in order to fully see the image.
No, I do not want it for a background. \
As the website is photography show room oriented, I am actually trying to find the best possible way to set-of every image/picture.
So I am thinking, one way is that the image to best fit the screen for best viewing result.