Specifying a width and height for all images allows for faster rendering by eliminating the need for unnecessary reflows and repaints.
Page Speed recommends:
Specify dimensions that match those of the images themselves
Don't use width and height specifications to scale images on the fly. If an image file is actually 60 x 60 pixels, don't set the dimensions to 30 x 30 in the HTML or CSS. If the image needs to be smaller, scale it in an image editor and set its dimensions to match (see Optimize images for details.)
Be sure to specify dimensions on the image element or block-level parent
Be sure to set the dimensions on the <img>element itself, or a block-level parent. If the parent is not block-level, the dimensions will be ignored. Do not set dimensions on an ancestor that is not an immediate parent.
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