The height and width should be included in the HTML. The reason being is that it creates the spacing on the page. If for whatever reason the img fails to load (and you've been a good boy and included an alt..the browser will show that frame (using the w and h provided) with the alt inside.
The main beneficial reason is preventing the "pop" effect. When a browser loads the page sometimes larger aspects such as img take longer to load, and if you have not specified the w and h in the HTML the browser will temporarily collapse that area thinking its not there. Then, when it finally loads everything pops into proper place.
This is especially annoying but still pretty annoying on a computer because you are going to click on a link and all of a sudden the page shifts down and you've accidentally clicked the wrong link.
