Will the following code make my css deal with images in the optimum way?

No, the size of the source image still matters.

The rule only means that the image will not display wider then that.
It is there mainly to prevent the large images from breaking the layout.

This will have no effect on the download time (browser is loading the original image anyway) and you are actually making browser do some little extra work to resize the image. Also different browsers will handle this task differently so be sure to check how your images look if they are being heavily browser-resized.

Further discussion and workarounds are out of the scope of this site.

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