You may have created a free-form email template within which you have re-sized images which you re-sized to make smaller within your image browser.
When you create am email campaign you are finding that when sent out, the images are expanding and distorting your campaign...
A: Re-size your images outside your account first, then upload them again as the re-sized version. You should then be fine.
Hint 1: when creating a Free-Form template always start with inserting a table of no wider than 600px
Hint 2: when using a banner image always keep the width to no greater than 600px