This image has not been edited from the default upload options in post editor:
It's too wide for the container, but the container will squish it. It should look distorted even on a large sized screen, and will be hard to recognize on a mobile screen.
The image again, with img-responsive and no size info:
This image is properly responsive. This adapts to the size of the container it is within on both mobile and desktop screens. It's powered by predefined bootstrap style "img-responsive".
Here's the image again, with img-responsive but no change to default width and height:
It doesn't seem to matter if width and height are defined. This adapts to the size of the container it is within on both mobile and desktop screens.