The Newsletter Pop Up plugin is extremely flexible in terms of the size of image you upload to it. The minimum recommended width is 310px (or 620px for Retina displays), and you can size the image to be any height. However, we do recommend scaling the height of the image to match (or be longer than) the amount of content on the right.
For example, if your embedded form is long, with fields for first and last name, email, and numerous different newsletter segment signups, you’ll generally find a vertical/portrait image will compliment the volume of content. On the other hand, if your form only listed a “Subscribe” title and an email field, you could easily get away with doing a landscape image instead!