When Creating an Image...

Specify ImageSize->Full when creating an image to make it display the full width of a notebook:

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When the Image Already Exists...

Wrap an existing image with Image and specify ImageSize->Full to make it display the full width of a notebook:

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Notes

Images with ImageSizeFull resize as the notebook is resized.

ImageSizeFull works for graphics as well as images:

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Click for copyable input
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Use a Scaled image size to make an image display at a fraction of the notebook width:
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If you give a Scaled value larger than 1, you can scroll horizontally to see the whole image:
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Specifying ImageSizeFull affects the display size, but not the image dimensions as returned by ImageDimensions.