If one specifies either the width or the height, but not both, the image will scale up proportionally. But specifying width AND height means those two numbers must be in the precise proportion, or the picture will end up stretched.
You are correct, it will scale proportionally, but it's my understanding (I may be wrong) the image will load faster if both are included. Mich, do I understand this correctly?
When I need to scale down an image I usually take it to a graphics program and resize it or change the height AND width by 1/2, 1/3, 1/4, etc... as needed.