Without the width and height designation, the image is allowed to be displayed in its native size, which could be, visually, far larger than intended. However, by setting the dimensions, a very large image (in size and bytes) has to be compressed to the specified size. This takes a bit more computational work by the browser as well as the time to download the full-sized image.
If the image *is* the same size as the specified dimensions, I think the byte sacrifice is in the number of characters of code to state them (which is insignificant).
I'm more interested in understanding the practical difference between "/download" and "/preview" for the image source.
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