When including an image as part of your survey header on the Style tab we recommend the below image sizes.
Header Images - we recommend images files up to 750px wide and 200px tall.
Banner Header Images - If you would like your header image to display like a banner that stretches across the entire page, we recommend image files 1000px wide and 200px tall. To accommodate this image size, change your Page Width (available under Style > Layout > Widths/Sizing) to 100%.
Learn about other recommended image sizes in our Recommended Image Size Tutorial.