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PageSpeed
YSlow

PageSpeed: Specify image dimensions

Overview

Specifying a width and height for all images allows for faster rendering by eliminating the need for unnecessary reflows and repaints.

How does your site score on this recommendation?

Page Speed currently only detects image dimensions that are specified via the image attributes. If you are specifying the dimensions via CSS, then you can safely ignore this recommendation.

Details from Google

When the browser lays out the page, it needs to be able to flow around replaceable elements such as images. It can begin to render a page even before images are downloaded, provided that it knows the dimensions to wrap non-replaceable elements around. If no dimensions are specified in the containing document, or if the dimensions specified don't match those of the actual images, the browser will require a reflow and repaint once the images are downloaded. To prevent reflows, specify the width and height of all images, either in the HTML <img> tag, or in CSS.

Summary

Images
Medium
Easy
97%

PageSpeed 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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