Putting in the dimensions reserves spaces for the images, so as soon as you load a page, it will be the correct height, even if the images aren't done loading yet. If you don't add the dimensions, then when you first load a page, it will be shorter than it should be, and when you start loading a particular image, the page height will grow to accommodate it, causing the jump effect. I'm not particularly bothered by it, but I guess some people would prefer no jumping.
Much more noticable when responding to a ping, say to reply #847 in a thread. Particularly if there are a lot of graphics on the thread. And as often as not, when the thread finally settles down, it doesn't end up where it's supposed to.
Now, I could stop loading the thread, read the post, then resume loading the thread, but the point being, there is a right and wrong way do do such things.
FGS