Any way to control the width and height images in email templates?

We have added a header image in our email templates which works fine and has a fixed width. The issue occurs when a user posts an image which is wider than the header, the mails body becomes wider than the header, we have added some CSS to it but it is not picking up. Is there a way we can apply the fix to the images in the email templates?

We are updating General Subscription Mail and the images etc. are in ${notification.HTMLBody} context object.