Final answer:
A responsive image is a fluid image that adapts to different screen sizes and resolutions, preserving the page layout as the browser viewport is resized.
Step-by-step explanation:
Ethan Marcotte described a responsive image as a fluid image that will not break the page layout as the browser viewport is resized. A responsive image adapts to different screen sizes and resolutions, ensuring that the image remains visible and visually appealing regardless of the device or screen size being used.
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