Preserve aspect ration for images in Calibre

I have converted my Word file to ePub in Calibre successfully, but when the new epub file is expanded or shrunk for various devices the images distort and do not maintain their aspect ratio. I fixed the cover, but I have jpeg images for each chapter heading and they squash down width wise to tall and skinny when the file is reduced in width for smaller devices. Could someone please provide some HTML code to keep the images at their correct aspect ration of 16:9 ?
Thanks