Jim Robinson has always said to add the width and height because the thread will load up faster. I do it like this.
< img width=356 src="http://www.analemma.de/images/articles/dicicco.jpg"heigth=303>
The only way I know to speed up handling of large images is to first convert them to a small image using some image processing program on your own PC, then uploading the shrunken image file to a web site, and setting your <img src=URL> to that uploaded, smaller, image file.
One other detail -- I would specify only one of height or width, not both, in most cases. If you specify both, and don't get their ratio the same as the height-width ratio of the original image, then you will force the displayed image to be stretched in one dimension or the other. Better to just display one of the two, height or width, and let the other default to whatever is needed to preserve the proportions of the original image.
Thanks muchly, wilco.