Certainly, education might help some of the problem, but it occurs to me that most people put their pictures in through clients such as Semagic, Visions, etc...
As such, could we perhaps encourage the developers of such clients to add a feature that would determine and insert the proper height and length parameters for a default picture scaled to 100% by default?
What I mean is, if the the client sees an image tag with no height or width attributes defined whatsoever, perhaps it can insert these attributes in ala Office typo correction?