Depending on your message, you might want to convey your message in a different form.
The first option available is an image as a picture is worth a thousand words. It will be resized in a fluid manner to fit the template it's put into while maintaining its ratio.
You can also display plain text. It can replace or go after a picture. For instance, you can use it to inform you guests about a particular event happening.
It is also possible to choose an interaction to provide a richer experience for your guests.
It is possible to open a text popup if you need to display a long text in a friendlier user interface for your guests.
Also available is a file download. This way, you could allow your guests to download a coupon for instance.
The last option is an external URL to get your users to if they click on your campaign. Beware that this option is only possible after authentication and will not be available on signup pages.
Of course, you can mix these to display text and a picture with an external URL interaction.