I am having the same issue. Please look at this tweak:
gurner I am using DW in such a way that I want to serve images stored in the media folders with a given height, not width: I am using DW as the content management system for a "Welcome screen" in our reception, so we can display images, messages etc; the screen is in portrait orientation, and I want the image only to take up a fixed chunk of the screen.
This seems like a solution. Would it be worthwhile?