I've been looking around for an answer to this one for quite a while now. I can't imagine there's no preference setting for autowriting image dimensions.
Use the image insert button in split or design view in order to achieve that. But you have to place the cursor where you want your image to go in the design view area. Done and Did!
I usually work in split mode for the most part. It took me a while to realize that different insert buttons will do different things depending on weather you're inserting in the code area or the design area. Glad to be of help.