Image h x w attributes don't affect the actual speed of your page load much, Dan. They do strongly affect the perceived speed to the user.
If the size attributes are included, the browser can leave a correctly-sized space for each image as the page gets rendered, even if the images haven't started to download yet. Then the rest of the page content flows in around the image "placeholders". (Images are always slower than text.)
If no image size attributes are present, the browser essentially ignores the placing of the images until the image files actually download, then redraws the whole page to add the space back in for the images.
This redrawing for the images means that text and other elements will move around on the page until all the images have downloaded and it has finished rendering. This gives the user an impression of a much slower page, since they can't start to read the content until it has stopped moving around. Done properly, the visitor can start reading the top of the page even while all the images lower on the page are still downloading.
So yes, obviously including height and width attributes for images is standard best practice for designing an effective on-page user experience.
Hope that helps?
Paul
P.S. As proof, Google thinks they're such a standard requirement that they have included a check for them as part of the scoring algorithm of their Google Page Speed tool.