Is the width and height necessary for images?

We are automating an email with products as content so my template needs to be able to accept images that may have variable sizes. For example, one is 190 x 250 or another is 250 x 190. In order for the images to not be stretched to fill 250x250, is it just as simple as removing the width and height from the img or can I fill in with "auto" or 100%?