Specifying image dimensions for site speed vs. responsive

[JannetteP]

I'm working on improving site speed from an SEO perspective, and one bit of advice I see often is to specify the dimensions of the images you're using so the browser knows the size of the image it needs to download.

However, I am wondering what impact this may have if the site is responsive? If you specify the large dimensions suited for a desktop browser, would you be forcing a mobile browser to use that sized image?

Has anyone seen dramatic improvements in site speed using the <picture>tag for responsive images?</picture>

Thanks!

Jannette

[Andrew-SEO]

Specifying dimensions of the source image does create a faster loading time as there is no resizing done in the browser. You should then use different sized imaged for different devices.

This is a good resource

https://moz.com/learn/seo/mobile-optimization

[ThompsonPaul]

To be clear - specifying the image dimensions on the HTML has no effect on what size of image is downloaded from the server. It serves only to specify the space on the html page that should be reserved for the image display when it does finally load. So it gives s the impression of faster speed by keeping the page from redrawing once the image dimensions are actually known by the browser.